Replace your pedals with dual sided pedals - one side is SPD, the other normal platform.

Shimano sells the A530 dual sided pedals. I've used them. They work really well. Available in silver or black.

On days you plan to ride over 80% of the time, put on your MTB shoes with SPD's and clip in. On days you plan to walk a lot, put on your regular tennis shoes then flip the pedal to the platform side.