All of those activities seem like they could work. Memorizing pi (“pie”) with number shapes and number rhymes might be fun. Alphabet peg systems can also work with kids. They might also like creating memory palaces for animals, dinosaurs, or cartoon characters.
I had started this topic as my daughter loves mnemonics too, and others shared their experiences too which is really lovely, I used it for inspiration too a few times. Maybe you can find some things to do here.