Flexible 3D Printing materials are a popular filament option, enabling stretchy, bendy, and rubber-like creations...
As with all filaments, there are many different manufacturers and variations of flexible filaments. The primary requirement for printing with flexibles is an extruder with a direct path that keeps the filament firmly in place from the time it leaves the drive gear until it enters the barrel of the hot end.
Popular brands like the NinjaFlex and Filaflex are great choices for when you want super squishy and stretchy items. Other material such as bendlay and soft PLA are great for printing objects that are bendable, but not very stretchy (such as timing belts). But all of these materials could be very useful to the resourceful tinkerer!