We are aiming to do the same in the next 6 months or so. We've been researching a lot on what to do.
Have you scuba dived before? I have read it's very cheap to get certified there and an amazing experience.
On a side note, what vaccinations do you plan to get? Looks to be a few irregular ones an average American should get. Unfortunately from what I've read, insurance doesn't cover it.
Food wise, everything I've read has said the street food is amazing and very very cheap. Basically a massive meal would probably be around $1.50, so most people just go that route.
Have you looked into hotels yet? I've seen some good reviews of very affordable hostels. If you feel you might want to spend a lot of time at the hotel, the resorts seem like any other tourist area.