@ fan of dan would love to hear your advice
going sun morning and leaving thurs afternoon. not enough time but this is what the wife agreed to 
You should have enough time to get in some scenery as well as have time to relax just don't try to do everything.
Did you mention you are flying in and out of Liberia? If you are that is much better than SJO as it is not as complicated to find your way there. GPS is worthless in Costa Rica btw.
The 2 most popular sites in CR are Arenal for the volcanoe and activities and Manual Antonio for the beaches, rainforest and activities. There are also beaches closer to Liberia in Guanacaste CR but I would go with Manuel Antonio. The beaches there are in the national park and are in front of these huge tropical forests.
Monteverde is nice but IMHO overrated