I'm partial to seated raises using a block or plate under the toes and use a heavy enough weight to get 25-30 reps and ON FIRE with acid.
This allows me to use LOTS of weight and get a stretch w/o stressing my spine. Otherwise I'd have to basically do raises with my heavy squatting weights to get a workout....nope.
Another option is on the leg press, but it feels weird to me and risky.