We have done something where the town was split in two. One half fought while the other half "marched", and the marchers threw on tabards and masks and NPCed in that first fight. Then reverse roles. Everyone got to participate, the lower levels had a challenge scaled to them as did the higher levels, there were lots of NPCs to simulate a real huge army and that epic feel let everyone come away with a very positive experience. Provided people are good sports about being conscripted for a short while, everyone's a winner.