Sea monkey / brine shrimp eggs plus tank (Rodbourne SN2)

Plastic fish bowl, USB powered air stone, food scoops, and brine shrimp eggs.

Additionally, you'll need salty water and food - spirulina powder.

1 litre of dechlorinated water (we used bottled) to 1 tablespoon of sea salt, and no more than a couple of teaspoons of eggs, I think! There are loads of eggs, so you can experiment. Keep out of direct sunlight. They eat algae, so you won't need to feed them often once the environment is established.

You will occasionally have to top up the water.

Our kid has finally lost interest after keeping them (and watching them reproduce) for a couple of years. Great for primary school science, learning about life cycles.

Or you can breed them to feed your fish...