I once saw some really nice little books that you could report all your shows and results in. I'm sure they came in a few different colours so could get a different one for each.
No idea where I saw them though and I can't find them online
Depends how grown up they are, but toys r us have these fab cuddly toy ponies that come with felt tip pens so you colour them in paint by numbers style. You then put them in the washing machine and start again! Big ones were £10, small ones £5, and they had different colours (pinks, purples and turquoise I think) so you could get them different ones. Saddles came off as well i think.
Honestly it won't matter how grown up they are I bought a similar thing for both of my sisters a couple of years ago, I think they had a cat and a dog complete with pens to colour them in - they spent all xmas afternoon colouring - They were 18 and 20!! PMSL