Turkey, I suppose, but there is not much that they could really do without getting the Russians to destroy them. I suppose they could make some waves in the Middle East in an effort to aid Rommel. They had the manpower, just not the modernized army.
Spain, as you mentioned, however, like Turkey, there is little they can get out of it other than Gibraltar, which they'd have issues taking anyways, and they were too damn spent to try it. A blockade would kill them.
Finland could have gone further all in with the Germans. Had they done so, Leningrad might have been taken in 1941. They only allied with the Germans to get land back, not to take any. This saved them in the long run.