Shesaysso - lots of recommendations for C America, but I guess it depends on your budget, tolerance of risk/discomfort and what you like to do on holiday.
Our holidays there have tended towards the low budget/lowish comfort end but obviously this has changed a bit since we had DD. There's also only so many hours you can spend on local buses when you have chilren!
One of our trips (pre-DD) was on an open-jaw ticket to Panama City and departing from San Jose in Costa Rica. This covered seeing the Panama Canal, bird/wildlife watching in Soberania National Park, Christmas with indigenous people in the San Blas islands, diving with whale sharks in the Pacific off Santa Catalina (although bastard sharks took a day off the day we were there!). Then New Year in the cloudforest before we crossed the border over to Costa Rica and did a 5 day trek through the jungle at Corcovado. We carried all our own gear and camped, but trip can be modified/made easier for families. In the jungle we saw snakes, tapir, wild boar, crocs, bull sharks, primates and nearly saw a jaguar.
We did the above in just over 2 weeks - am quite shocked when I look at the itinerary now!
Both of the above 2 countries could be done at a slower pace on their own - they both have nice beaches and both would also offer adventure/family-orientated activities (Costa Rica more so but is more expensive).
Guatemala is an amazing place to visit - no major beach stuff but it has lakes, mountains, jungle etc and is a great place to pair up with Belize as a 2 country trip. Guatemala is also extremely cheap (we averaged around £5 for a hotel room) and the tourist transportation network is good as there is a well-trodden tourist/backpacker route around the country. Belize by contrast is pricier and has a very low-key transport network (buses slow and sparse, but internal flights cheap and frequent). It does offer a lot of beach life though and the snorkelling is amazing.
Most flights to Central America are on the pricey side, but do vary slightly from one country to the next, and most involve transiting through the USA which takes time. On our last two trips, we've flown direct to Cancun as it's a lot cheaper (if you aren't bound by school holidays) and is quicker. I think Thomson are starting direct flights to Liberia in Costa Rica next year - worth looking out for as this will make accessing the country much quicker and probably cheaper.
Sorry for rambly essay - please shout if you would like info on anything more specific. 
PS have visited Nicaragua, Honduras and El Salvador too, but didn't enjoy particularly. The latter two in particular scared me and was very happy to leave!