Why not get them (or make) some gingerbread men and get them to decorate, if the weather is bad, not too messy! have you a paving slab path? I let my dd's have chalk drawing competitions on the slabs, keeps them busy for a while and easy to wash off. Will think of more Food is easy! Usual party things, pizza, sausage rolls etc! It'll be as hard as you make it.
My dd is a bit younger but loves threading beads to make necklaces - for her 4th party in a few weeks we are decorating party bags (felt bags from Baker Ross/plain cotton drawstring bags for the boys) with various stick on gems, stickers etc, the girls are also making necklaces and bracelets( got the beads and elastic thread from hobbycraft and am going to give them their own beads in a plastic cup) and boys are decorating gingerbread men (from Tesco. I'd second having a look at Baker Ross or have you got a pottery painting place near you if you don't want to host?
Ds and his friends painted plates last year for his 6th birthday. I bought kits from Tesco which contained the plate and a set of paints so they all had one each. It was great and they all took them home as their party bag filler!! Also, they made their own pizzas for their tea too!! I put a selection of toppings out so they could choose what they liked and leave what they didn't like. One boy refused to eat his because he didn't like the things he'd put on it!!