V.K.
Hi! I live in Cleveland now, but grew up in Erie, PA. It's not your typical big city with a ton of well-known hot spots, but it should be. There is tons to do there.
You've got the Presque Isle beaches. Beach 6 is probably one of the best. It also includes volleyball courts. The beach is free! http://www.presqueisle.org/
Waldameer Park. It's an amusement park. Right at the top of the beach. Admission to the park is free and you can buy tickets for rides or a ride-a-rama if you prefer. There is also a waterpark there which is optional (and is charged separately) http://www.waldameer.com/
There's the new casino there. It's not near the beach, but it's not too far of a drive. It has horse racing and slot machines. No table games yet. (But probably not for the whole family) http://www.presqueisledowns.com/
If you like boats, there's the dock. You can ride the victorian princess. http://www.victorianprincess.com/
You've got the Erie Zoo. It is less spread out than the Cleveland Zoo and includes a special kiddie zoo... http://www.eriezoo.org/ And if you have a membership to the Cleveland Zoo, it will work at the Erie Zoo too.
Of course there is also Splash Lagoon... http://www.splashlagoon.com/ The kids absolutely love that place.
There is also a children's museum down near the dock, if you decide to ride the Victorian Princess, that would be nearby.... http://www.eriechildrensmuseum.org/
I can probably think of more. What I like about Erie is that nothing is usually more than a 15 minute drive away... It might take 25 minutes max to get from one end of town to the other.
Oh, and there are plenty of sports teams there too such as the Erie Seawolves if you like baseball. (A lot cheaper than going to see the Indians) http://www.seawolves.com/
Oh and some of those things are seasonal, such as Waldameer, some of the beaches may be closed, and the baseball season may be over, so you may want to consider going before Labor Day, if you can.