I agree with the others. Since you have your own vehicle. Scenic drives may be the best way to go. The day tours are great for those who have no other way of getting around (besides buses) and even pretty good for those that don't mind leaving the rental idle, but they can get kinda pricey. If you have your own van and you go for a drive all you have to pay for is gas.
A couple of minor corrections:
1) The town near the Orosi valley is PARAISO. If you go by there be sure to stop in to explore Lankester Gardens.
2) The Cerro Del Muerte stretch of the PAH can be very dangerous once the fog rolls in (usually later in the day) but it doesn't get its name from any fatalities (though I'm sure those occur there as elsewhere in CR). The name comes from early settlers who died from exposure while crossing that pass long BEFORE there was ever a road. I haven't driven with Handy but I think Papi's advice of driving carefully up there is a good one for everyone to follow.
3) The "chair lift" is the Aerial Tram (
http://www.rainforestrams.com) just on the other side of Braulio Carrillo Park that others have referred to. If you drive there yourself be sure you've reserved a tour ahead of time.