How about a trip to the local library a couple of times a week.

Check local theaters for summer movie camps (usually like a buck or so a ticket).

Still indoors I realize, but at least away from the house, a little adventuresome, change of pace.

Check out public pools. Even if you only go once every other week, they'll be thrilled. We joined our city's community center for on $50/month for the whole family which I though was an absolute deal (outdoor pool park with slides and lazy river,etc., indoor pool, gyms for basketball, game room, exercise room for adults, indoor track). We had no idea how affordable it was until we really investigated. Daily passes to public pools are usual pretty cheap, depending on how elaborate the facility is.

Since you are in Pennsylvania instead of Texas, outdoor parks might be weather tolerable. Investigate free or half-price museum days, free tours of factories are fascinating.

We're near a city and just discovered a hotel that shows family movies at its pool that anyone can come to for free, and you can swim while you watch (how cool is that for kids).

You have to be a bit of a detective to find things, but there are lots of things available for little to no money to do if you work at it a bit.

You don't have to entertain your kids constantly, but getting them out of the house a few times a week at least will go a long way to keeping them from utterly dying of boredom. It's good for you, too.