Wow, all three choices are great places. I personally loved Yellowstone.

All three are great places, but there's no place like Yellowstone for it's combination of great scenery and abundant wildlife. Lots of good hikes. The whole family will love it. Book your rooms early as the in-park lodging fills up.

We would welcome you to the Yellowstone area! There is a lot to do for a family in this tri-state corner of the world. You not only have the wonders of Yellowstone Park but Jackson Hole is just a few miles south in Wyoming. We have a beautiful area called Island Park south of Yellowstone in Idaho. It's a great place for camping, fishing, floating the river, etc. Rent a beautiful log cabin very reasonably!

You will find very reasonable lodging, eating, etc rates in this area as well. Fly into SLC, which also has a ton of family oriented sights - Temple Square, Olympic sites, Park City, Lagoon theme park, etc - and then drive up into our area either through Wyoming (beautiful drive) or I-15 North (takes about 5 to 6 hours). Feel free to stop in Idaho Falls along the way for great lodging and a beautiful riverwalk area.

Hope this helps a little bit!

A few years ago our family did RMNP and it was great. We stayed at the Summit View villas and they were beautiful, brand new and 5 minutes from the RMNP entrance. We hiked there and explored for about 4 days. We white water rafted one day during that time too. Estes Park is a nice town and is easily walkable. From there we drove to Steamboat Springs and stayed in a condo we had rented through vrbo.com. We went to the hot springs, a rodeo, horse back riding in the mountains followed by a campfire dinner. The kids went down the alpine slide as well. It was a great vacation w/lots to do and beautiful scenery. I don't think you would be disappointed.

I've been to all three of these areas and feel all would be enjoyable for you and your children. I personally love the mountains and the West. Rocky Mountain Natinal Park is probably my all time favorite vacation destination. There are fabulous family hikes. It's beautiful pristine Wilderness, but a much more manageable sized park than say Yellowstone and Grand Teton National parks (These are wonderful too and offer more diverse wildlife and geology -- the gysers, bubbling pots, etc. at Yellowstone -- Buffalo, Moose, and Antelope that you don't see at RMNP, but I usually spend more time in my car in these areas vs. RMNT). At RMNP that trip to the Safeway for food/supplies is pretty quick and you'll see tons of elk, birds, ground squirrels. On our last trip we saw beaver and the big horn sheep too, but I don't see Big Horn Sheep every day or necessarily on every trip. There are waterslides, mini golf options, souviner shopping in Estes Park too that kids love. Obviously you get this type of thing in West Yellowstone and Jackson Hole too. The Niagra Falls area, in particular, is very touristy, but quite appealing to most kids your children's ages (you can spend a lots of money there pretty quickly) -- kids love the falls, the ferris wheels, the lego exibits, -- places like Ripley's Believe it or Not, the Titanic museum -- this list goes on and on. My trip to Toronto was without kids, so I don't have a good feel for more kid friendly attractions in the area. I loved the city, though. It really felt like a cross between a North American city and a European city.

For a different perspecitve, I asked my 12 year old who has been to all of these places what he would recommend and told him you have three boys ages 9-12. He said that they are all great and worth doing and that you'd probably want to at sometime in the future plan a trip to each. He, though, said that he'd pick Niagara Falls. He thought is was just so cool to get so close to the Falls on the Maid of the Mist boat ride and that the whole place was really fun. This, though, is the trip he's done most recently of the three, so is probably the one that is most in his mind. These are all great kid places, so I'd pick what appeal to you most.

If you go to Yellowstone, you can get Grand Teton National Park as a bonus. http://www.nps.gov/grte/ Just South of Yellowstone, it is a much smaller park with good hikes in stunning scenery. Float trips on the Snake River in the park or whitewater trips outside the park are fun things to do as well.

The trips West are significantly longer than what you did this year. If you have the time (at the right time-not the busiest time) to do it right, go to Yellowstone/Teton.

There are benefits to Toronto, etc... It provides some variety to the great outdoors trip youve just taken. There are so many great things to do in Toronto and Niagara Falls. You can make the drive there part of the fun. For instance, drive to Manitowoc and take the ferry across Lake Michigan. It doesnt save you time, but is relaxing and my kids always love it. One of the ways, take the US route, and you can stop at Cedar Point near Cleveland. We were in Toronto this June. We had 9 and 11 year old boys, along with our 6 year old daughter. The boys especially loved it, and they love have a big city experience now and then. Just riding subways is exciting for them. We love baseball, so we took in a couple Blue Jays games. The CityPass covers most of the destinations you will want and is a real bargain. We loved our day riding bikes, etc...on the Toronto Islands.

But-theyre great options-cant lose!

Wow, these are some really nice suggestions. Thanks everyone for your responses.