Can you recommend any exercise (swimming, biking, clearly not running) that would be good for drop foot?

Footdrop: Consider range of motion and exobiology exercises to prevent contracture. Also, swimming is good. Wear afos to allow adls and improved gait pattern.

Drop Foot Exercise: Drop foot is caused by a neurological injury to the nerves that innervate the muscles in front of your shin. Since that injury may not be complete damage, doing exercises that consistently stress the shin muscles is recommended (build whatever muscle you can). Swimming, biking with stirrups, elliptical, slow walking (with a AFO brace), yoga, and pilates are common recommendations done without limp.

Physical therapy: It'd be nice to know whether it is complete or incomplete. Incomplete means there is some residual foot function, while complete is when there's no function whatsoever. Also, some reversible causes can be successfully treated, whereas irreversible won't be. Start with physical therapy that will evaluate the degree of your foot drop and they will be able to tell the best course of action.