My five year old had been learning about money at school, so for Christmas we got him some toy money. He was absolutely delighted but become disturbingly Scrooge-like, counting his money over and over again, and declaring himself rich.
The money was good for playing shops, and trying to work out change. It turns out that calculating change is a quite tricky, requiring subtracting and then splitting the result into coins.
I decided to try something a bit more abstract: asking him to figure out the coins required to give value of 1 - 20 pence. To make it slightly more interesting, I said he should trace around the relevant coin to draw it, which turned out to be harder than the actual calculations.
Initially he didn't seem to understand the question or thought it was boring, so he said he was going to do his own thing, which turned out to be exactly what I'd asked. He seemed to find it pretty easy, and normally found the smallest set of coins for a given total.
One exception was 17p where I think he realised that he needed to change the 1p in the 16p total to a 2p, but forgot to include a 5p. When he realised the total was 12p he added 2ps and a 1p to make up the total.
A couple of months later, completely unprompted he made a maths sheet for me, which was very similar to our activity. I think it was based on something they had done at school. He made the sheet more interesting by drawing pictures of the items we were going to buy (biscuits, mushrooms and ice cream), and all the coins we could use to buy them with.
Even though the sheet was meant for me, he did the sums himself. I was impressed he made each of the totals using the fewest number of coins, starting with the largest coins and working down.