Pocket operator (particularly the KO) is the single thing other than maybe the OP-Z that I think has enough creative utility relative to its price to not feel like glorified furniture.
Any used Akai device. Depending on how old I would say get them the new standalone device called the mpc one+. If the price is too steep look for a used midi device from Akai.
Honesty, a used cell phone would make a better pocket operator.