Narrowing down Central/South American Destinations

My fiance and I are planning on honeymooning for 14-16 days across the last week of December (Xmas) and the first week of January.

We’re seeking 60/40 adventure to beach/pool so we’d like to do a big city or two and then somewhere relaxing.

We’re enthralled with the idea of Mexico City and all the cultural things we can do there (+ the food and drink) and going from there to Oaxaca and Huatulco is on the table, but we’re just not that excited about what happens after Mexico City.

Alternatively, we’re considering going to Brazil and being in Rio for Xmas and or NYE, which seems very special. From there we’ve considered going to Buenos Aires and maybe even Uruguay. This weekend we’re going to time these things out to see whether a plan like that is practical.

We don’t really like the jungle or hiking, though we’re willing to walk for hours in a city.

We’ve loosely considered Nicaragua, Costa Rica and to a lesser extent, Colombia.

If you can help us strike a balance between trying to do to much but also seeing as much as we can that would be much appreciated!