The Cape Cod Rail Trail is GREAT for cycling. It goes past Arnolds where they have a really nice Airstream set up as a bathroom stop.
The Brewster General Store is worth a visit, and there's an ice cream shop next door. Also check out the Cape Cod Museum of Natural History in Brewster.
If your truck is a 4X4 and meets some other requirements you can get a beach driving permit ($50 / week.) Air down the tires, drive out to Race Point, and pull up a piece of beach.
A visit to Provincetown is an interesting social experience.