Remember, with one foot clipped in you can still pedal the full circle as your foot that is clipped in will pull the pedal up. So you can even "dangle" one foot if you need to and still make progress for awhile. Not very pretty, and I find it quite tiring, but it can be done.

Shimano has pedals that have clips on one side and platform pedal on the other. So you can ride in tennis shoes if you want.

My hubby has the Crank Bro pedals, and they don't look like they could be used without being clipped in to me.