One of the largest waterfalls in Europe. 5 SFR entry. The way they've built the viewing platforms lets you feel like you're in the falls. Very impressive and worth seeing!
Awesome in Autumn as well. Visit from north side via Neuhausen to walk down to the falls. Take boat tour 1 to central rock. Basel to Schaffhausen via BW ticket and the local train to Neuhausen.
Awesome in the summers. Take a boat to the centre of the falls. Quite relaxing. From Basel take a BW ticket and reach Schaffhausen and then S-Bahn to Rheinfall station.
South bank of the river has free parking, but paid entrance (5 CHF). North side is free entrance, but paid parking. I liked the north bank better thanks to the boardwalk along the river.