I have two weeks to travel in December, and I can't decide where to go. I went to Belize last year (twice actually, because i loved it so much!), and would happily go again, but the world is a big place...
What I am looking for is 1)somewhere where I will feel safe traveling alone (26 F) 2)spanish speaking so that I can practice 3)not touristy but with a tourist infrastructure (i know, i want it all!) 4)somewhere i can split my time between water and land activities.
I would be grateful for any suggestions!

Certain parts of town in Puerto Vallarta Mexico fit what you describe.

La Paz on the Baja was very nice, less tourist infrastructure but great water activities on the Sea of Cortez.

I have not been yet but Playa del Carmen, Mazatlan, and Zijuatenejo are all on my to-do list and I would personally feel comfortable going to any on my own.

Maybe Honduras would be a good choice for you. Check out our photos and travelogues if you're interested:

http://flickr.com/photos/staceyholeman/collections

The islands won't give you much Spanish practice, though, as English is commonly spoken.

Guatemala is gorgeous but doesn't offer as many water play options. Keep us posted!

Though a bit more touristy than Belize, which we loved, Costa Rica offers a lot of options.

I'd second the nomination for Costa Rica; we've been 3 times and can't wait to go back. Check out La Fortuna/Arenal, Manuel Antonio, and the Osa Peninsula. Several women on this board have traveled to CR alone and loved it.

I would highly recommend Costa Rica as well, but would have to disagree with the previous post about being more touristy than Belize. I found it to be the other way around, but just my opinion. The people, places, food, and country in general are incredible and very safe. I would suggest Arenal/La Fortuna for your inland, the active volcano at night is something that everyone needs to experience, then Manual Antonio or the northern beach area for the sea.