Well, I'm sure you have reasons for limiting us to Belize or Costa Rica.... ;-) but from a mileage standpoint, if you're using CO/NW, you can fly into South America for the same amount of miles.

IMHO, I wouldn't spend 35K to go to Belize because it's not too hard to access Belize from Cancun by bus. if I had 35K to use, I would go somewhere farther south. But that's just me.

You may want to ask/check on the LatinPass board because many people there know a variety of spots in Central and South America, albeit maybe for just a few hours. My hunch, though, is that more people there will know Costa Rica and not Belize-- that's my case, so I can't fairly advise of one over the other.