Mostly foam, reinforced with various things. But not that wimpy white stuff that blows apart if you look at it funny. Heavier blue or pink foam in the 25 or 40psi density. EPP foam is also very popular since it has the ability to "bounce" and give. It's the same reason the full-contact glider and zagi guys use EPP foam.

Wing spars are usually fiberglass rods. The glass rods can bend and flex with impacts rather than break like wood or carbon fiber will.

HDPE (high density polyethelyne, aka "cutting board material" is often used for parts of the fuse, since it can bend and flex to absorb impacts, but is actually hard to break. Light weight aluminum angle is sometimes used as part of the fuse as well, it's light and strong, flexes a little, and can be bent back in to shape if it gets really mangled, or cheaply and quickly replaced if it's not fixable.

Tail surfaces are coro, since again, it flexes and takes hits well. I've landed to find 3-4 prop cuts through a vertical stab, but the plane kept flying and the tail stayed on there, and kept the plane stable. A bit of tape between rounds, and it was back in the air.