Lush bath bombs are ace, the boys love lobbing them into the water and watching them fizz away. The drawbacks are the price, potential skin sensitivities and I'm pretty sure that some of them contain soya lethicin, which both boys are intolerant to (and I can't be bothered enforcing the "no drinking bath water" rule).
Ingredients (makes more than 10 small bombs)
Bicarbonate of soda - 2 cups (235ml)
Cream of tartare 1/2 cup (60ml) OR citric acid 1 cup (115ml).
Coconut oil (or another type of oil that would be good in the bath, sweet almond, canola, baby oil) - 1 tablespoon
Essential oils -20 drops of each one
Food colouring - a few drops
Water, coconut extract OR very strong herbal tea - I have used chamomile, but I have read about green tea, and I bet peppermint would give you a zingy bath - 1-3 tea spoons.
In a large bowl, mix the dry ingredients.
In a separate smaller bowl or jug, mix the liquid and the oil.
Add the liquid to the dry ingredients and mix well.
If it isn't sticking together well, add a bit more oil (or water, but water makes the alkali and acid react, so it steals the fizz from bathtime).
Lavender, clary sage and lime is nice, but you can really play around with scents, depending on how much you want to spend on the essential oils. Just lavender would be fine and you can pick it up quite cheaply at local pharmacies. I'm quite frugal, so I didn't buy chamomile oil but used chamomile tea instead.
Some recipes suggest using cornflour/ cornstarch, but I read that can encourage yeast infections so I ignored that!
The mixture should hold together when you press it into a mould - silicone bun cases worked well as it was easy to press them out: mould, pop out and place somewhere to air dry at room temperature.
They will be fragile, but as they dry they will feel more robust.
When they feel dry, pop them into a container and use them at will! Probably best to use them within a month though.