The obvious answer would be homemade sweets or spice mix. If your friends are crafty you could fill with coloured or unusual beads for jewellery making. If you have friends who love to party you could fill with glitter to sprinkle in hair when going out (ensuring it is ok for body use). The list really is endless!
Another quick idea is to 'hang' jewellery within the jar. For this idea I have kept it all very simple - a jar, a pair of earrings and some ribbon. To be able to hang the earrings I attached them to a piece of cling film which I cut to fit the jar. As the jars are quite small I wouldn't use a gift tag but either tape a message on the jar or you could use a waterproof marker pen directly onto the jar so it becomes a keepsake.
The jars are available in places like Ikea and can be as little as £1-£2.