The first thing I would do if I were you is to extend your trip for a few extra days!
Just kidding!
Most areas are safe as long as you use common sense. Take travelers checks, cash or credit cards if you don't mind the extra 3+% they will charge you for "foreign purchases." Fishing is great, but I don't have guide recommendations.
Some ideas for your trip (in no certain order):
1. Snorkel Hol Chan/Shark Ray Alley
2. Rent a golf cart and explore the island.
3. Eat at the Reef, Waraguma and El Patio. Get caye lime pie at Carumba, Jambel Jerk or Blue Water Grill. For a nice romantic dinner, take a water taxi up to Capricorn or Portofino.
4. Find a nice barstool and enjoy a cold drink.
5. Take a ruin tour on the mainland.
6. Walk around town and enjoy the view.
7. Walk down the beach and enjoy the view.
8. Find some live music to enjoy.
9. Find a nice beach chair to relax in.
10. Slow down and enjoy your trip!
AmbergrisCaye.com 
