In an ice-cube tray (or shaped mould)..
Fill 3/4 full with normal plain water.
(I use a syringe or dropper), put 2 drops of food colouring in each.
Add 1 drop of a flavoured essence that will release as it melts. [optional]
Use a toothpick (or similar) to give each a quick stir to make them evenly coloured.
Add a couple to bathtime. Or an outdoor pool.
Kids will have lots of fun trying to catch them before they melt, they are easy to see and smell great.