Summertime there are charter and party fishing boats headed out of Indian River Inlet. Many years ago I went with some of my buddies and others we didn't know on the Judy V. My 9 lb. 12 oz.
Bluefish won the boat pool.
You can take the ferry between Lewes & Cape May coming or going.
I have also been on a fishing boat out of Lewes which was quite good.

Zwanandael Museum in Lewes has shipwreck artifacts. The icecream store with turquoise paint on Rehoboth ave next to the boardwalk, features homemade icecream...40 flavors...including chocolate with bacon, which was featured on a segment on the food network.

Lewes is a cute town and fun to wander. There is a state park at Cape Henlopen on the beach. There may be hiking trails there. I don't know for sure. There is a big outlet mall nearby if anyone is into shopping.

There is also shopping along Rehoboth Avenue IN Rehoboth itself as well as just doing the boardwalk there. There's always the possibility of a day out in dreaded Ocean City, too.

Assawoman State Park outside of Bethany Beach is a great place to bike and kayak. Marshland, unpaved roads, a fire tower to climb, piers to crab and fish from. From route 26 go to the library, take a left past the bridge, another left or two after a while, and eventually you'll get there. It's biking distance from town.

We vacation in Bethany Beach every year. Bethany itself has a very cute downtown boardwalk area. Lots of decent restaurants. A good place that both kids and adults can eat in is the Cottage Cafe. Bethany Blues used to be really good but IMO has gone downhill the past few years.

Not far from Bethany, in both O.C. and also in Lewes, you can do one of the "Pirate Adventure" ships for the grandkids. The kids get to dress as pirates and go out on the bay. The cast at both have the kids interact and shoot water cannons, haul up a pirate treasure chest and other things. Google Pirate Adventure Ocean City or Lewes and you'll find it. (Different companies run each).

There are also very nice, clean amusement rides in Rehoboth. Not expensive there and the grandkids would enjoy. They also have that in O.C., but it is louder, busier and much more expensive (not to mention dirty).

The Bayside Skillet is the one place in O.C that might be worth the short drive. Very fun atmosphere and really good omlettes, fresh chantilly cream, terrific peanut butter and jelly crepes for the kids and other home made goodies.