I have had great success in past years with keeping away the bunnies.  I sprinkle a good bit of ground cayenne on the leaves & they don't seem to like that at all.  I don't know if this is something you'd want to do with spinach though.  I generally do it every few days & after any dousing rain, at least until the plants are mature enough to survive a bunny attack.

I grow a variety of peppers, beans, tomatoes, snap peas, squash, cucumber, onions, carrots, radishes, and asparagus, as well as basil, cilantro/coriander, dill, and parsley.

I haven't had a squirrel problem before.  I have heard, however, that squirrels are delicious.  :)

Herring405