I don’t really want or need anything, fortunate as I am, but I have to provide a few sub-£20 things for a few people lest I get accused of being ungrateful/snobby/etc.
Trigger bell so you can keep your hands/fingers on the bar/brakes
3-way hex wrench for bolt checks
Pedal covers to save your house/car
Grip refill
First aid kit
Helmet hook
Might be necessary to stress that it’s that particular model you want, as non-cycling folk seem to consider everything interchangeable (i.e. trigger bell = any bell).