How Can I Make a Bouncy Platform?

I'll continue Fattie's thought: Study up on Physic-materials. Under the collider options in the inspector panel, you'll find an option for Material. Import the standard asset bundle for Physic Materials, and put something like 'rubber' into it. From here, you'll want to adjust the properties till you get something that works for your specific application, which means doing some study at The Physic Material Section.