For where you'll be you won't need 4wd. Sure there are lots of dirt roads that go to great places, but you need to know the country better to get involved in that. The road from LS to Arenal is nice, easy to drive. There are three ways to get there, and only one of them is curvy. If you go via the drive around the lake, it's pretty, and tranquil, and not too curvy. I wouldn't want to go to Arenal for only part of a day-there's too much to do, and you need at least 1-3 days.

A nice beach near LS is Esterillos, but don't swim near the river mouths because of the rip tides and maybe a wayward crocodile. The surfing and bodyboarding in the area can be outstanding. You might enjoy visiting Playa Hermosa, just north of LS, mainly to see big waves (normally) and watch surfers.

You can do whitewater rafting tours on the Rio Savegre, leaving out of Quepos every day. If you drive a little south of Manuel Antonio the scenery changes a lot, and there's hardly any people. Playa Dominical is pretty and you can park between the coconut palm trees.