Nori rolls are fantastic and very cheap to make. If you don't know how to make them, you could try and get a bulk deal on them from a japanese takeaway (most shopping centres have one). Then just chop them up and arrange on a plate.

These and a frittata are always good as you can have heaps plated up before hand to fill the gaps when waiting for other food to warm - or to hand around first thing when there are not many people around yet so you aren't letting hot food go cold.