In Lewes there is Cape Henlopen State Park. Great place on the water. Former military base that served to guard the bay during WWII. There are miles of bike paths in the park, including a cool boardwalk part through a tidal area. So many people ride there that they have those bike repair stations with tools and air pumps. Extremely popular campground there as well. You can ride to Rehobeth almost completely on paths and bike laned streets. There is a also a trail system that was recently expanded to reach right into Lewes itself. Some of it goes through a marsh area and is very pretty. You can also take your bike on the ferry and ride to Cape May, NJ. The center of Lewes itself can get clogged with traffic in the summer, but people are used to seeing bikes. The free beach is also decent.