Hey. I have fished Mexican and Central American waters for quite some time. In 3 weeks you will be right in the middle of Snook, Bonefish, Tarpon, and Permit paradise. Ascencion Bay is a top-notch bonefishing destination. Snook will be staging in the lagoons and mangroves as will tarpon in the 30-40 lb range, although larger fish to 75 lb can be found. I wouldn't even sweat bringing your own gear. Guides are plentiful, they'll know what works, and they'll have the equipment to do the job. Alot of guides market to fly-fisherman, but most all of them will also spin fish. Since you're trying to kill two birds with one stone (I know what it's like, the wife and I fished during our honeymoon too) my suggestion is to hire a flats guide for a full day trip and chase anything that swims. Like I said, you should have a shot at 4 species of fish from that one area. See what your hotel can set up or suggest for you, but also do a little foot-work on your own to compare to the costs they will give you.
Here's a good start:
http://www.igfa.org/certified_captai...n.asp?id=38215
http://www.ascensionbay.com/

Tight Lines.