Presuming first that your are in a walking boot for an ankle or foot problem and you are permitted to remove the boot for bathing and sleeping than just about any non-weight bearing (NWB) exercise would be fine.

First, remove the boot for exercises. In addition to the short knee extension exercise suggested by Kay, you could do a longer knee extension from a full sitting position. Knee flexion could be done lying on your stomach and in standing NWB on the bad foot by bending your knee. Exercises from the hip, like lifting your straight leg towards the ceiling as you turn from front to side to back to other side is also good to keep leg muscles strong. Do 10 - 30 reps each then turn to the next position. Do exercises 1 - 3 times per day. Stop if anything does not feel right. And of course ask your physician about what you are permitted to be partial or full weight bearing and when it will be time to see a physical therapist.

Good luck on your recovery.