>>10131789More functional than fun, but I like to fill small hidden interior gaps with round studs instead of plates. A basic example, two round studs can replace an interior 1x2 plate, or more advanced you can do things like replace two stacked 2x2 plates with two stacked round studs placed in the center of the 2x2 space like a tiny column. Since you'll never see them they can be whatever color is cheapest, although it is cool when a stud layer peeks out as a little detail. They are extremely cheap because they come in a million colors and are painfully common, esp in Friends varieties. I get mine from local bins but I imagine you could set yourself up to use this technique for the rest of your life for a few bucks on bricklink.
Pic related is the only marginal example I have photos of. About a third of the hidden interior here is studs. On the left you would see the exposed sides of a few studs under the white angled plate to look kind of like an intake if the lighting was appropriate. The bottom of the model has some overt stacked-stud detailing which is another technique you could have fun with if you cached up on studs.
