you could stand on a step, the front half your foot on, the heel half off and flex yourself up and down. make sure you have a bannister or something to grab on to so you don't fall backwards. just go up and down, up and down, bending your knees.
I do leg lifts and situps every day. both of those exercises work the core because I believe the abs are as important as the legs.
Squats...lots and lots of squats...lunges to the front are also good.
What I have found with experience as I work with a personal trainer, is that riders naturally have stronger hamstrings (back of the leg muscles) than front quadriceps (front of the thigh). The hamstring is naturally stronger anyway just due to the purpose behind them but riders have better developed hamstrings than the average non-rider.
Through squats, the last 4 months with a barbell of sort ranging in weight between 15-45 pounds (as I got stronger), the trainer had me doing dead lifts in a squat..essentially doing the squat with the weight. I can't tell you how strong my legs have gotten. I am now leg pressing 197 pounds...the first day we did the leg press exercise as a starting point, I was at about 63 pounds and that was hard.