honestly found belize pretty weird. we ended up having to sleep in a chinese supermarket one night. the cayes are great, very relaxed very fun. there are some mennonite communities who live in the jungle which is strange. agreed re some of it being a bit sketchy, big ports.
belize - we enjoyed orange walk, the ruins at Xunantunich, caye caulker. placencia was the chinese supermarket, best avoided I think.
guatemala:
tikal - absolutely must see, its amazing. also get a kayak and potter about the lake.
lago de atitlan was great. dont fall for the scams about the ferry though, we really nearly did...
antigua was a bit so so.
the caves and the waterfalls at lanquin were literally amazing. I have no idea if they still let you do a tour in a bikini and trainers with a candle though. swimming one handed into a massive cave holding a candle was awesome.
livingstone was fine, not much to right home about.
we hopped into mexico, i'd also recommend San Cristobal de las casas and palenque. both really interesting.
i could write for days about CR - i was a volunteer english teacher there. what type of thing do you want to do?
it is safe, easy and a great place to start. If i were you, I'd spend three weeks there. much much easier with a family. you could really get under the skin of the place and visit lots of different bits.