Well, your first time, huh? I hope it turns out as magical as ellydognme's trip - see her report!
Ok, I don't know anything about Villas, but heard it's nice and was going to stay there on one trip, but plans changed.
As to other questions:
Eating: Don't miss Caliente. Have some Gorditas or Coconut shrimp. More reasonably priced for lunch than dinner. Portions for lunch, at least, are big. Breakfast - Estelle's was my favorite, and Lily's a close second. You can get coffee and banana bread/juice at Ruby's for very cheap. Lot's of other good places, I'll leave those recommendations to others. I liked the food at Fido's as well - but some say they're overpriced.
Day trips: I'd do as much as you can on your budget. Go see Sandy at SEArious, and ask her to hook you up. The Manatee trip was my favorite - snorkeling, beach BBQ - it's $75, but it's all day and lunch is included as well as a stop at Caye Caulker. Willie and Ricky are both excellent guides.
Snorkeling - never did Mexico Rocks, but Hol Chan shouldn't be missed. Again, Sandy can take care of that - it's $20 or $25 for a couple of hours. Jeffrey is a great guide for that, but whoever they have will be terriffic, I'm sure.
Where to party? Where not to party? Start at Fido's and ask around where the hotspot of the night is. My personal favorites were Dockside on Wednesday (ladies' night), and Barefoot Iguana on Friday night.
Have an awesome time!