Pinholed ping-pong balls with a bit of water in them, but still floating, barely, plus one or two intact balls above the surface. Found that years ago from one of the aquarium mags for Oscars, and tried it for the mbu as well. Just like a toddler in a high chair or playpen, "watch the people retrieve the toys from the floor". Don't use any that crumple in adding water - if crumpled, the fish may be able to eat the plastic (I used a syringe and needle to add water, but not everybody has access to those).
Attention-getting device - rather that always wagging against the glass closest to me, he would bounce the balls out of his (yes, open-topped) tank. Usually with his head, but also by spitting water. Quite similar to Archers, hunter-predator puffers are master spitters, it is a snail IDing technique.