I would recommend Guatemala. I originally had a trip to Guatemala and a trip to Panama planned (I was living in Costa Rica at the time) and I loved Guatemala so much, that I canceled my Panama trip and went back to Guatemala a second time!
I would recommend starting with
Antigua Guatemala. Most hotels in Antigua have airport pick-up from Guatemala City for a reasonable fee. Spend 2-3 days here. Walk around the town, enjoy the great restaurants, take in the colonial sites, take side trips to San Antonio de Aguas Calientes, the water volcano, mirador de Santa Cruz, and many others!
From Antigua, head to
Lake Atitlan. I kid you not, it is the most beautiful lake in the world. Spend 1-2 days here. Take a small boat (a bit pricey, but worth it!) to San Antonio. Visit the places where the women make the local fabric and enjoy unparalleled views of the lake from all sides.
I ran out of time on my trips, but I would recommend spending the last few days visiting the ancient Mayan pyramids of Tical.
Good luck!