I couldn’t really mention the OP-1 (and sneakily the OP-Z) without mentioning the PO Series (eBay Link) from Teenage engineering too. I have the first set, and may consider the next lot when I’ve got some cash in my pocket.
These are, without a doubt (or at least without a doubt in my mind anyway), some of the most fun, and also innovative, tiny little music making machines around. They’re entirely pocketable, and their diminutive form disguises the massive amount of enjoyment that can be had from them. Whether you have one, two, or 6 of these they will be great fun, both to learn how to use them, and to create some sounds from them.