We were in Costa Rica (Peninsula Papagayo area... from LIR airport) in March and chose to rent a car. While I don't recall exactly what the class was (Group L, as I recall), we got a diesel Montero.

I don't know about using coupons, but I do recommend renting a 4x4 while there. The roads we traveled were very rough in places (potholes, most notably) and would have been most uncomfortable if riding in something lower.

Good luck... PM me if you need more advice...