Not directly a sandbox creature but how about a not hostile creature that can be traded with? (Like the villagers in Minecraft)
It could look similar to the Gremlin creature and could be spawned with one random trade offer (x Wood <=> y Stone │ x Ores <=> y Seeds │ x Ammunition <= y coin (goldsink)) for the first player that trades with it.
These creatures should only spawn rarely though so their impact on the player economy is negligible.
I think this would make some nice encounters in the wilderness of B<.