Bucks County, PA
If it's not too brutally hot, take a bike ride. The 12-foot-wide towpath of the Delaware Canal, a National Heritage Hiking Trail, runs for 60 delightfully flat miles from Easton to Bristol.
You can also cross the Delaware on one of five bridges near New Hope, and ride New Jersey's own towpath along the D&R Canal. The path runs along the other side of the river for 25 miles, from Bull's Island, New Jersey (8 miles north of New Hope) to Trenton.
Both towpaths are scenic, and filled with small towns that once served barge operators when they pulled up for the night. You can stop at any of them for brunch or just to wipe the sweat from your brow.
The most popular route: Start in New Hope, the unofficial capital of Bucks County, and ride up to Centerbridge. Cross the bridge to the New Jersey side, and ride through Stockton and Lambertville. Then cross the bridge back again to New Hope.
Another well-trafficked route starts in Washington Crossing State Park. Ride up to New Hope, cross the bridge to Lambertville, ride south to Washington Crossing, New Jersey, and cross the river back to your car. For a good breather, rest your legs on the deck of Faherty's, a restaurant right on the towpath by Washington Crossing overlooking the canal.
In New Hope, it's also possible to tour the canal by mule-led barge, just like you could in the waterway's heyday. The difference is, now you must pay for the experience, and listen to some pretty hokey narration.