My son built the catapults with minimal help from me. The instructions were clear, the pieces all fit together well and with minimal fiddling, and it even included a tube of wood glue. After some time spent waiting for glue to dry and stickers to stick we were ready to start flinging things around the house.
There really is nothing that is not suited as a catapult target. Chairs, books, people's legs. We could fling apple pieces to the chickens. We could fling cookie pieces into each other's mouths. So picking a target is a win-win endeavor. But if you are looking for a particularly fun time, try building a tower out of blocks, stocking it with guards, and then blasting away with small stones. We made our tower as tall as possible, with lots of Astro Guards perched on precarious landings and balconies.
Here we can see the progression of the tumultuous siege of Fortress Astroman. This was a good forty five minutes of stone flinging but in the end the catapults were victorious and the crew headed off for dinner. Of course I forgot to take a picture of the final ruins of the tower, we were too excited at the time. Flinging things around the living room and knocking down towers is an afternoon that's hard to beat.