i would recommend not implementing features that are fees or taxes, because those would just be widely hated and avoided. the frog pond is a great idea because it rewards you for giving up large sums of potatoes, it never at any point is a punishment. *and* it's optional.

actually, to tell the truth, my reaction to the frog pond was that the frog token prices should be higher. i think if it's just kept stocked with new items to grind toward, it'll probably balance out. only time can really tell.

also town hops for potatoes is a great idea. though i guess it wouldn't act as a sink unless you paid more for the hop than what you'd get out of visiting a new town. it's still a good idea, though.