I occasionally pick up my fretless, do a couple of slides, play the bass solo from Lost Horizons (Mo Foster in the first MSG album), noodle about for a while, and then put it back down to wait for the next occasion.
One even rarer occasions, I pick up my Ashbory, but between its miniscue scale, being fretless, and rubber strings, it's almost impossible to play anything even remotely in tune.