Middletown has an old-time roller skating rink called Doc's Family Fun Center where you can spend some time on the hardwoods rollerskating around. If you're looking for more exciting things to see and do, I would suggest you take the 15 minute drive into Hershey or Harrisburg where you'll find miniature golf courses, shopping malls, Hersheypark, Zoo America, museums, etc. Both Hershey & Harrisburg have a lot of fun activities for you to enjoy.
Middletown is nothing really special and it's definitely not the type of place you want to plan a vacation to. If your passing through or staying nearby, you can cruise on through and see if there is something there that interest's you.