As far as I know, overnighting in the falls parking lots is not allowed. Four Mile Creek is a really nice state park, especially if you get one of the sites with a good view of Lake Ontario. It's about a 20 minute drive to Niagara Falls State Park from there, and your Four Mile Creek receipt gets you into the falls parking for free. If you have the time to spare, I'd suggest at least a two night stay at Four Mile Creek so you can relax a bit after a day at the falls. We found spending one day on the US side and one day on the Canadian side really gave us the best of both "worlds". The evening light show is also fun to see.