What do you plug into the inputs on an audio interface?

You could plug any line level device into them. That could be something like an external analog mixer, a drum machine, an electronic drum kit, a keyboard, a looping device, an effects unit, and actually guitars and bass as well. Guitars and bass guitars do actually put out a line level signal, but we are used to hearing them after they go through pre-amps that are specifically designed for them, so while it will probably work, you may not like the sound of it. You could also use additional microphones by sending them through an external microphone pre-amp before sending them into the 1/4" line level inputs.