I haven't been there in years, but there are lots of old lovely churches around, many damaged from the frequent earthquakes. Lots of smaller villages in the area, and nice little markets. How are you getting there? I went by bus from Guatemala City (scary trip) and shared a seat with a pig. Possibly it's improved. Should be lots of festivals and parades around Christmas. Anyway, it's a lovely area, fairly high up so not really hot. If you speak Spanish well you won't have any trouble filling your days with things to shoot (photographically speaking). It's a coffee-growing area, but not sure when they harvest. Lots of B&B's around because of the many language schools in the area.
Unless you like big white beaches and western style resort hotels, I think you might find Belize boring by comparison. Unless of course Guatemala decides to invade it again. In any case, Belize isn't a place I'd get too far off the beaten path.
Have fun!