If Arenal's erupting, go up that way (Fortuna). We had a motel on a hilltop facing the volcano, about 7-8 miles away, and from our front porche we could watch wads of lava speed down Arenal all night. If Arenal's not erupting, though, the area's nothing special.
If you want more of a Carribean flavor and want to hit up a cool national park (Cahuita) head to the east coast. Here's a page.
http://costa-rica-guide.com/Natural/Cahuita.html
Cahuita's cool, Puerto Viejo has a nice end-of-the-road Jamaican feel, a lot of back-packers end up there. There's stuff to see, and horses wander wild through the streets. It's possible to drive down to the Panamanian border from the east coast, but I wouldn't do it -- there's a pretty scary border town there.
Can't think of anything else right now, but I'll keep thinking.