I've made a little game to occupy my mind in a non-challenging way:
The Never-Ending Sum
christianp.github.io/the-neverendin…
It's like a colouring-in book, but for people who like doing sums.
I'm interested in ideas for other kinds of sums.
In reply to @christianp
Oh, forgot to say: it uses some disgracefully new browser features, so wont work on anything more than a year old