Here's one I use on longer courses for those who have really like fire. It's designed to teach people about fire lighting and fire management.
Take a dead (not rotten) log, about a foot long and about three inches across and burn it to nothing whilst still making it last as long as possible.
You do it with only a knife and a sparker. No external tinder and no other fuel or kit.
The method basically involves breaking it down a bit, feathering bits and getting the fire lit and then lighting the rest.
Yes, you could match stick it all, but it would not last long. But equally, you could make bits too thick and it either not light all through or not all of it burn to ash.
It's a fun, repeatable challenge. Have a go.
Maybe do it with a mate at the same time and see which one can make theirs last longest.