I have been to Ambergris Caye twice in the last year and think that it is a great place for bonefish and tarpon.
You take a water taxi from Belize City to Ambergris for about $20. Fair to good hotels cost $50. There are a goodly number of guides who will take you out for $125for a half day, $175 for a full day Food is good but a bit on the high side.
There are always a lot of bones but the biggest I have caught isabout 6 pounds. The tarpon fishing is great if the wind is from the right direction but takes a full day.
If it is not the high season, I go for 10 days to two weeks and thenpick which days I want a guide. There is little else to do there if your your not into shorkeliing or diving.
If you're interested, email me and I'll give you a recommendation on hotels, guides, etc.