My kids were always arguing about who is allowed to ride the small trike on my backyard, so I decided to make a trailer with a seat for it.
Time spent was three evenings, couple hours each + painting.
The cost of materials is unknown as I only used whatever I found from the garage; metal cut-offs and an old piece of plywood.
The amount of time my kids will argue about who will be driving and who will be sitting on the trailer; unlimited.