As far as leg presses go,try doing one set of 100 reps..........stay on machine until you`re done no matter what.
Another scenario is to do 50,40,30,20,and 10 reps while adding a plate to each side on each set.
As far as squats go,I do squat at times, but I go light on the Smith machine, and squat in rapid succession without locking out at the top...........hurts like hell and no back problems.
Another good one is to do a pre-exhaust super-set of leg extensions,immediately followed by light front squats on the Smith machine...........killer!!
Then you could pick 3-4 exercises and do them all in a row with no rest until the last exercises is done.....rest a few,then repeat for a few more cycles.