We went to the Mopan River Resort (http://www.mopanriverresort.com) in May 2003 for a week. It was an amazing trip. Lots of included cultural day trips, good food and drinks, and reasonably priced. Would definitely go again.
An aside - the owner is wanted by the IRS (big time). You can read about it here: http://www.escapeartist.com/efam26/bacon.html During happy hour, he's happy to share his stories about how he screwed the US gov't. Very amusing.