What to do on the Cape for the next few days

[Jim in CT]

My mother-in-law is at the house for a week, she wanted some alone time with the kids, so my wife and I are heading to the cape for a few days. We wanted to be near an indoor tennis facility, so we are staying in Eastham. Any suggestions for activities, restaurants that are open (we prefer casual, clam shack type places, most of which I presume are closed). Can we see seals in the winter? What are good beaches to take a long walk on near there? Lighthouses to see (she loves lighthouses)?

Thanks in advance.