I loved Costa Rica. I lived in San Jose for a summer, which is not the most memorable place to be. Manuel Antonio/Quepos is amazing! The area in between Manuel Antonio and Quepos has very nice hotels and evens some gay bars (bar Tutu). I stayed at La Mansion, which was expensive, but very nice.
Arenal in also amazing. I stayed at the jungle lodge, which was very nice and inexpensive. We had a great view of the volcano and there was a nice breakfast included. My friends stayed at the Tabacon resort, which is much more expensive (I believe its a 5 star resort). You can buy a day pass to the Tabacon Springs which is about $40 and totally worth it. It has tropical rivers and waterfalls that are heated naturally by the hot springs. There is also good hiking and zip linning (canopy tour).
Monteverde is also beautiful, but you can do many of the same eco-tourism activities in arenal, so if you have limited time, I would think you could skip it.
We also went to isla de tortuga from San Jose through a charter tour. It was very nice. (This is not to be confused with tortuguero island).
We also went river rafting on the rio reventon. I never made it to Limon on the carribean side, though i hear it looks similar to other carribean destinations.
By no means am I an expert; there is a lot to see in such a small country.