Went on my honeymoon to Belize..actually San Pedro, Ambergris Caye which is a 20 minute airplane hop ($120/pp R/T) from BC. Or you could take the 90 minute ferry over which is an adventure in itself (c. $15/pp each way). Once you get there you can walk around the area, shop, lots of good local places to eat but not so overrun as the touristy areas around the port. We stayed a few miles up the Caye in a private villa so we did all our snorkelling and diving out on that end.
If you want something a bit more off the beaten path, go to Caye Caulker which is about a 10 minute hop from BC by plane (c. $80/pp RT)....great swimming, snorkelling, and home of the world famous Green Lizard bar.
BC itself is safe as long as you don't get off the beaten path...can be rough in some places. I would recommend going with a pre-booked tour guide rather than finding a local if your going to the ruins, jungle etc. There have been isolated cases of highwaymen in remote areas.
Look around thios site for ideas:
http://www.tripadvisor.com/ShowTopic-g291961-i1457-k1690441-Water_Taxi_or_Airplane-Ambergris_Caye_Belize_Cayes.html