Welcome!

Long-term parking at EWR costs $18 a day. There are some privately operated long-term lots near the airport that are less expensive. A Google search for "newark airport parking" will come up with a lot of options, check out the private facility reviews carefully.

If you want to visit Niagara Falls, I'd recommend you do it between South Haven and NYC, it's right along the way if you transit through Canada (an easy 1 day drive) (as is Niagara Falls to NYC). It's also not that terrible a detour if you stay on the US side all the way. Doing it on the way from NYC to Miami would be a considerable detour.

I personally don't see much "touristy" about Pittsburgh, to me it's just "another city" - but all cities do have redeeming qualities.