Rehoboth is a great town for walking.  Even if you aren't in shape, there are many places to take short walks and find things to view.  

First of all, the boardwalk is great.  The mid section is busy, but at both ends it is less busy.

The beach is good to walk on, especially at low tide (go by the tifeguard station on Wilmington Ave and the boardwalk to check the halkboard for the tide, temperature, and wind info, or check it online).

The whole town is easy to walk.  Pick up a map at the visitor center and just walk.  You can walk for miles on the town streets.  They are full of trees and friendly people. In the fall or winter, you will probably have the streets to yourself.  

 If you like a Good Walk, drive or walk on the beach north to the Gordon's Pond State Park beach.  From there, you can walk all the way past several WWII gun sighting towers and on to the point where the Delaware ocean coastline makes a hook and becomes the Delaware bay coastline.  You can also walk on the Gordon's Pond trail for a mile or so.  It goes out to the inland pond, which is a temporary home to some migrant birds in season.  The Gordon's Pond and ocean area is also dog-friendly all year round if dogs are on a 6 foot leash. 

The Junction-Breakwater Trail is a great way to get from Rehoboth to Lewes, if you have the time.  It is offroad for 2/3 of the way (the last third is on Gulls Neck road into Lewes).  It follows an old rail line for most of the wqy, and is a good  surface.