I am all for easy craft ideas and this is the easiest. So stupidly easy, in fact, that I can't believe it's taken me this long to figure it out. I had both a packet of foam stickers and a handful of old ribbon rolls/bobbins that I'd chucked in our Useful Box after they'd been denuded of ribbon. And one day... bam.
Stickers, meet ribbon roll.
Add some paint and it's a cheap and easy-to-hold toddler stamp.
The thicker the foam sticker the better in terms of stamp quality, but Dear Boy was happy with these. The vague shapes of sea creatures appearing on the page was greeted with all manner of exclamations: "Octopus! Seahorse! Fish! Shell!" That's a success in my book.