X2 Four Mile Creek State Park on US side. Right on Lake Ontario, wooded, very popular. Note not full hookups but there are electric sites, water spigots for filling your tank, and a dump station. Very close to Ft. Niagara which has costumed interpreters, museum, musket firing, etc. right by the lake.
It is about 15 minutes north of the falls, straight shot on the Robert Moses Parkway. A bonus is that if you show a dated New York State Park campground hang tag at the Niagara Falls State Park, you park there free. (Saves $10).
The NF State Park has had a multi-million $ renovation the past couple of years and looks great. We were there in July (2014.) From Parking Lot #1, you have an easy walk across the Rainbow Bridge if you want to take photos from the Canada side looking back toward the US side.
While in that area, it's fun to go to nearby Lockport on the Erie Canal. Canal boat rides, museum, newly-restored locks, towpath for walking along the canal.