In reply to Eam1:
I'd recommend flying fly in/out of Cancun, as you'll probs get it cheaper than to Guatemala City, Belize City, etc, plus the Yucatan is a safer for a warm-up, before heading south. Or at least start in Cancun and try find a return from further down.
The Yucatan is magic; Chichén Itzá, Uxmal, Tulum for its location more than the ruins, then Palenque, San Cristobal de las Casas, etc, then depending on how much time you've got, you could spend time in Guatemala (Lake Atitlan, etc), then across to Belize and back up (via Tulum), or continue further south. My pivotal memory of Belize is the entire country plays reggae from top to bottom, from dawn to dusk.
Try dodge the crime-ridden cities and spend as much time in the countryside, however transport links inevitably demand the former, hence travel well before dark and be smart, etc, though it's always half luck. You certainly can't do worse than my friends, who got robbed within twenty minutes of flying into Guat City!