I'd strongly recommend Costa Rica. It's small with public transport and has fantastic landscape and wildlife. If you're coming from Guatamala make sure you see Arenal, the live volcano; Tabacon hot springs nearby (make sure you have water shoes with you),; Monteverdi which has cloud forest and wildlife; the Peninsula for surfing and beaches. Be careful in San Jose, I normally avoid it if I can. Not much to see there. Local people are very kind and pleasant AND you can drink the tap water. Food is very good too.