Here are some random thoughts on Isla...

Overview:  The island itself is pretty small, but not tiny.  There are some cars (mostly taxis & the cops) and people also get around by golf cart, scooters, motorcycles, & bikes.  It is a pretty chill place - unlike mainland Cancun, where often the goal is to get wasted &/or hook up.  Not that these things are off the menu, it's just not a 24 hour hormone-driven hootenany.  There are plenty of places to drink, and you can usually find a dance party or two, especially on the weekend.  Mid-week it is a lot more chill.

Money:  Generally, just about anywhere on the island, you can use either pesos or US dollars.  It seems that people prefer pesos over dollars, but I don't really think it matters.  If you pay in dollars, you get change in pesos (with the typical standard exchange rate of $1 to 10 pesos).  There are ATMs on the island (the 7-11 fee is around $3, which is much cheaper than the ATM on Hidalgo) & places that will change your bucks for pesos at the going rate (usually hovers somewhere between 10-12 pesos per dollar). One of these places is located near the square on Hidalgo.  If you can't find it, ask just about anyone & they should be able to point you in the right direction.  Bring sunblock. It is costly on the island to buy it & it is even more costly to skip it & deal with the inevitable burn that you will feel no matter how many margaritas you consume (this is, of course, based on my own extremely scientific research).  I bring along a stack of singles for tipping. It's easy for cabs & small purchases, especially if you're not used to the different peso coins quite yet.

Beaches:  There are beautiful beaches around the island, but the biggest & most popular is Playa Norte.  It's very shallow & warm & you can either chill in the sand or rent a chair w/ or w/o an umbrella/palapa for a few bucks from the many restaurants/bars along the beach.  You will probably see some euro-ta tas, too.  Topless is optional. 

The international language of finger pointing:  You can get by with limited or no knowledge of Spanish, but it is certainly appreciated if you at least try..."gracias," "por favor," "hola," etc....  If you take cabs & don't speak Spanish, you may have a bit of difficulty, but don't sweat it.  Most folks are kind, patient, and excellent at figuring out hand gestures & poor pronunciations.   Don't be afraid to chat people up, but if you're limited to English (or some other non-Spanish language), don't be too surprised if the conversation is short.  After all, as uncomfortable as you may be speaking clumsy Spanish, they are likely feeling the same way ('re in their Spanish-speaking country).  Don't hide your smile under a basket.  You're on vacation!

Eating & Drinking:  You can purchase groceries & liquor & stuff from one of the grocery stores.  There's not a ton of selection, but it everything you need is available.  There are also a bunch of "mini-supers" around for soda, water, beer, etc.
The restaurants are mostly all great, with prices ranging from supercheap (roadside taquerias sometimes run out of the people's house with crazy animals all over) to pricey but not blow your face off 15-30 for a main course.  Most fall in the middle.  This isn't going to be one of those places that gets you bragging rights for how little it costs (like 17 cents for a beer in Malaysia or Turkey or whatever), but you won't feel assaulted upon re-entry to your post-vacation life, either.   I could go into a list of places that I love, but you are best left to your own devices. I've never had a terrible meal.  And, if you are so unfortunate, stroll down the road & try a different place.  I don't have a particularly sensitive stomach, but I've not gotten sick from eating fresh fruits, juices, or vegetables.  Most (if not all) places wash everything in purified water.  Ice & bottled water are also purified. 

Getting there (and leaving...sadly):  You get to/from the island by ferry.  Transportation to/from the UltraMar ferry dock in Puerto Juarez can be booked in advance (there are links on the Cancun airport website or you can do a search for something like "Cancun airport Isla Mujeres ferry transfer").  The UltraMar ferry is new-ish, very clean, not at all bumpy, and often offers live music on the upper deck (with the lower deck being enclosed & air conditioned).  The cost is about $14 round trip.  Tickets can be purchased at the dock.  Note:  it is the same price to buy a roundtrip ($14) as it is to buy 2 separate one ways ($7 each)...if you lose things like I do, you may want to just buy a one way & then another on the day that you return (which you can purchase from a small kiosk right at the ferry dock on the isla).  The ferry runs every half hour.  There will likely be porters that load your bag on & off the boat.  I've seen people become alarmed when someone grabs their bag...if they are dressed like all the other workers, don't be.  They are simply loading up the boat.  When you arrive, the porters may come right out & ask for a tip if you fail to do so.  You are under no obligation to tip, but I usually give them something small (like a $1 per bag). 

Getting around:  Cabs are inexpensive & plentiful.  If you are in town or just got off the ferry & need a ride, there is a taxi stand right next to the ferry dock (to the right if you are looking at the dock from the island).  Wait in line for the next cab, give them your destination & ask how much it will be.  The prices are generally standard - they may tack on a tiny bit if there's luggage or groceries that need to go in the trunk.  In dollars, I'd say a ride should never be more than $6, but is often closer to $3.   You can rent bikes, scooters, & golf carts.  I've only done golf carts - it is a fairly simple process.  Whether or not you are seeking to rent or not, you will be presented with many options.  Repeatedly.  Pick a place, negotiate a price, & you are off.  They will likely want to see a driver's license (they may keep it & return it when you return the cart).  I've read that you should always pay in cash, & not leave a credit card because you are on the hook for any damage to the cart & they may charge your card.  I've only had positive experiences with the golf cart fellows, but keep your wits about you when driving, parking, etc.  Always lock it up with the lock that they provide.  If you are staying in centro, you will have no problem walking to restaurants, bars, Playa Norte, etc.  I have found myself walking around that area of town at a wide array of times...4AM?  Yes.  11PM?  Yes.  8AM?  Yes.  I can't say that I've done this alone &, as with ANY place ANYwhere, you should probably use the buddy system.  But, overall, I've felt quite safe. 

 Carnivale: The weekend before Lent (aka Mardi Gras in Louisiana-speak) is Carnivale on Isla Mujeres. It's not widely advertised, in fact, you might only find out about it when you check in to your hotel. Crazy party hounds, take note, this is not for you. Isla Mujeres' Carnivale is a very family-friendly affair. There is a formal parade on Saturday evening, but in practice, it seems like people just decide to throw a big speaker on a truck along with some folks in costume and drive around whenever they want! It's wonderful. They will be performing all weekend long (and into the week). Definitely make sure to check out the Zocolo during this time for all sorts of interesting local festival-related events. 

 I am by NO means an isla expert.  For a small island, there's so much to explore & discover.  It is an ever-changing delight.  It reminds me a lot of Caye Caulker in Belize, if that helps anyone.  I'm happy to try to answer questions (with the disclaimer that I can only speak to my experience) at  Happy Isla-ing!!