Take a look at the Arenal area of Costa Rica and also Monteverde. The Manuel Antonio area has nice beaches inside the park and lots of wildlife, but would probably be more touristed than what you're looking for. If you really want to get away from it all, explore the Osa Peninsula.

Belize is awesome but it's not known for stellar beaches or cities, though there are some interesting small towns and villages. If you have 2 weeks or more, it combines well with Guatemala - fly into 1 and out of the other. Happy planning!

I am currently spending a month plus in Guanajuato, Mexico a most delightful Colonial City in a excellent mountain area to explore.
I next will be a month or so in Zihuatanejo my favorite beach side community. Small enough to relax but with all the convinces you need.
PS I flew into Guadalajara as I had a credit with Alaska but Aero Mexico or United into here is much easier. I am going up the coast after Zihua to Manzanillo & Puerto Vallarta with lots of stops then flying home from PV.

I'll suggest Nicaragua. The Pacific coast has some nice beaches. The colonial city of Granada is lovely, although, quite a few tourists. Easy access from there to several volcanoes.

Ometepe Island is awesome. Two volcanoes, and beaches on the lake.

We didn't make it to Leon, would have loved to. Another colonial city, I believe less touristed than Granada.

Southern Pacific Zone in Costa Rica fits everything you are looking for, except the city part. If you really wanted some city time you could plan a couple of days in Jan Jose at the beginning and end of your trip.

Look south of Manual Antonio if you want to avoid crowds. If you really want to get away from it all, head to the Osa, but the area in between is pretty extraordinary. Have fun!