Developing Leg Strength Off The Bike

Nimhbus - I hope i can help....
Squats are your main strength developer, as they obviously require the abdominal brace to activate more, and they actually require the glutes to work as a prime mover - as well as a stabiliser. (bear with me here). The Leg press is fine, but often means the "core" is not used as much - and more importantly, when the quads and glutes are loaded (picture the "tucked-up" position, before you drive the legs out), you are placing UNREAL amounts of strain on the lumber extensors/spinal column.
O.K. Now hit the step up. No it's not comical - its the more functional move out of the bunch - as it involves single-loading the legs.
Make sure that when you do these, you are using a step that forces you to lift to the same height as your pedal stroke. You musn't lean forward too much, and please don't begin with heavy weights! Squeeze the glutes through this whole movement, as you'll activate your small abductors as stabilisers (which is crucial for core stability).
Cable Wood Chops (from low to high) strengthen the core in such a way that is similar to being tucked on the bike, and pushing down on the crank. You go from a half squat - rotated - to extending/driving up and across the body.
I don't know what shape you are in, but elastic, dynamic energy is the icing on the cake here... plyometrics are king, but should only be completed if your knees, hips and ankles are up to the job.
Skipping is a good start - to see how the joints respond - and it builds excellent power in the calves.
If all is well, you could try jumping from one lunge position to another: get into the final phase of a lunge (with both knees at 90 degrees)then drive up and out whilst rapidly switching your leg position. These are often called dynamic Split Squats or static bounding.
Oh shit. I've got carried away... I wish i could take you through a few things and offer you my training opinions. I fucking love this shit.