Yes, I know that sounds weird. Well actually it is a little weird, but I have to admit I'm rather taken with our miniature eggshell planters.
It is actually something I saw in a book - Teeny Tiny Gardening by Emma Hardy - and which I have shamelessly copied.
I love it because it's so simple, yet still gorgeous to look at. In her book Emma uses pastel coloured eggs. I did think I might dye mine with food colouring, but then decided I prefered the plain old biscuit-coloured hen's eggs.
This makes a great children's garden activity too. Although the eggs are a little delicate for most pre-schoolers, older children should be fine. Also, their smaller fingers are a positive bonus when it comes to planting.
I part-filled the empty eggshell with a little compost - you could sieve it to make it finer but I didn't feel it was necessary. Also, Emma recommends adding drainage with some pin holes, but my eggshells seem to self-drain quite effectively.
The one issue you might have is in finding something to place in the shell but for me this is the best part: you can ask the children to look for tiny self seeded plants. It makes them really observant in a garden, peering right down on the ground. It's easy for us as my Breedon gravel pathways are the perfect environment for plants (and, sadly, weeds) to sow and grow, which means I'm inundated with tiny specimens. We used apline strawberry, Alchemilla mollis, alpine dog violet and a forget-me-not. However, there's no reason children couldn't put some annual weeds in these eggshells too - there are plenty around, many are very attractive, and there's not the worry of them spreading when they're growing on your windowsill.
You place the roots into the pot, fill around with compost, pressing down gently as you go, and then finish off the top with a little moss (again, gathered from pathways).
Given the size, these are very much temporary planters, but it's easy enough to pot on the plants when they have become too large (although not the weeds... obviously). You won't even have to worry about getting them out the pot: if they get stuck you can just use the Humpty Dumpty method.