I'm thinking someone should pay me to be a travel adviser, or something to that end, but maybe I will get paid back in street phad thai and good karma for all this writing I am doing.
B asked about belize/honduras versus costa rica/nicaragua which I would say are two very worthy choices to choose between and led me to think about what all the countries are like in central. I'm planning on getting to writing mini-guides on those countries that I have not yet covered but I figure I'll start with a what we might call a country briefing. I'm starting North and moving South, try to keep up. Oh and these are my opinions, nothing more.
Mexico has just about everything. Rich in culture, diverse in landscape, I can't really cover it all here. But my general feeling of méxico is, 'GUAU!' It is big, it has states, and the food in INCREDIBLE.
You could spend months or years exploring it: surfing, diving, mayan ruins, climbing volcanoes, visiting pueblitos, getting yourself lost in churches, meeting the indigenous people, swimming in lakes, exploring canyons. Chiapas is fantastic, the Yucatan (and associated states) is divine, and D.F. will blow your mind.
My soul lives in Guate. Highlands, black beans, incredible textiles, gorgeous colonial cities, TIKAL, and the warmest latinos on the block, what more can you ask for? Did I mention the coffee and chocolate?
For a cultural shmorgasborg, postcard beaches, and lush jungle Belize takes the cake. It is small, accessible, easy to get around, and full of friendly faces. Belize combines lots of outdoor activities with a carribean attitude, plus just about everyone speaks English. It isn't as cheap as other countries, but you can do it on a budget. Most people go for the diving, end up exploring a cave or two, and maybe some nice ruins. Don't miss the Marie Sharps.
Considering the current political strife, Honduras might not be your first choice. Honduras is in a word: strange. The catrachos are different from the other Latins and the Bay Islands are a world unto themselves. Copán has lovely ruins (but is more Guatemala than Honduras), las Islas Bahia have some great diving (and it is CHEAP), I've heard other places are great, but I haven't seen much. Most people stick to the islands and head on to the next place. If you want to get off the beaten track and see something new it could be a glorious adventure, but I am not making any promises. Combined with Belize it could be a cool diving vacation (there are direct flights between Belize city and San Pedro Sula).
Can't comment too much. Heard the coast is a great place for surfing. Pupusas seem motivation enough to check it out, but then again most people don't travel just so they can try the local food.
Could the Nicas be any nicer? Seriously. My best best best friends all live in Nicaragua. Traffic is usually caused by cows. Nicaragua is considered the next big thing though the economic crisis may have slowed that for a bit. If you want to experience almost everything Central has to offer (islands, volcanoes, diving, surfing, jungles, colonial cities) you could stay in Nicaragua and see almost everything. Boasting two of the longest coast lines in the region, two colonial gems, and decent infrastructure, it combines the variety of Costa Rica with the wildness of the other countries.
Costa Rica has grown on me. The people are unique and possess a subtle culture that will crawl into you heart and find a home, just give it time. If you like wildlife, getting outdoors and varied landscapes you will be blown away by Costa Rica. Not as safe as some people will have you think, it is still welcoming and lovely. The tourist infrastructure makes doing things a breeze, but there are still relatively undeveloped pockets, they just require 4 wheel drive and a bit more patience. Make friends with the Ticos, they know the best places and love to have a good time. Throw Nicaragua in the mix and you have an interesting balance of two very different cultures, the opportunity to do more outdoor sports than you can imagine and a hundred possibilities for adventure.
My Panama experience is limited to Bocas Del Toro, and I dug that. Panama City is rumored to be quite and adventure, David and the highlands come highly recommended. Plus there is always the added temptation of trying to reach Colombia overland from here. Send me a postcard if you make it!
So there you go, brief, succinct, and obviously biased. Hope that gives you some insight.