I have never seen people of any age being refused entry to anywhere but the Thai law was changed a few years ago putting the drinking age up to 20 years old. I do not see any reason why she would be refused soft drinks. People take their kids and even babies to restaurants which serve alcohol, but as long as the under 20s do not drink alcohol they are not breaking the law. Thai laws are very strict when somebody gets caught. You can read up about Thai Laws on tripadvisor in the facts on the right hand side if you are using a computer to read this. It sometimes does not show on mobile phones.
Long tail boat trips are things haggled for by the customer and the boat owner. Each one may offer completely different prices. I would recommend using the Orange Flag Express boats for journeys on the river which only cost 15 baht per person (about 40p). these boats are very useful to visit the grand Palace, Wat Arun, and other historical sites. You can go for about an hour all the way north to Nonthaburi for the same low price. There is an interesting market there a few hundred yards straight ahead on the right hand side of the road, as you get off the boat at Nonthaburi, which is the last stop .
For these boats you do not get tickets in advance. They are sold by a lady conductor who comes around rattling a metal ticket box when you are on the boat. Just say your destination and have small change ready. It is just like going on a bus years ago when they had conductors.
The longtail boats are only necessary if you want to do a trip around the small canals called Klongs. However these boats are like the equivalent of going in a taxi rather than a bus, and you may be charged thousands of baht instead of the 15 baht that you pay on the orange flag boats.
I think a 17 year old would probably like Khaosan Road where lots of young people and backpackers go. There are lots of market stalls selling souvenirs, backpacker type clothes, hippy style stuff, silver jewellery, hair braiding, massages, tatooists , friendship bracelet sellers, etc.
If she likes markets then I also recommend Chattuchak Weekend Market, which is in the north west part of Bangkok and it is one of the biggest markets in the world. Chattuchak is open daytime but the Khaosan market is best in evening.