Played a rather cool, Bank Robber game you may be able to adapt. We had a day with very few players last year so the marshall pulled this out and it was extremely fun. Key thing is that its quick games, repeated over and over, not one long slog. Fast pace keeps things interesting as you each try to outdo the last team. Time the games and the bank robbers who last the longest 'win'.
You need a 'Bank' we had a 3m x 3m square building with door and largish windows (knocked up of basic ply, usual site building style). However pallets, cardboard, pile of tyres or few well placed logs could build a suitable 0.5m barricade that the robbers can hide behind.
Game has robbers, hostage (normally acting as marshall) and special forces.
Robbers (2 or 3) go to the bank. No regen, 1 mag only of whatever they normally use, full auto. Based on robbers being reckless, but not expecting trouble. So you only have the 1 clip with you...
Special forces regen some distance from bank so they don't just "regen swarm" the robbers but plan. Special forces are limited to single shot, since they are the best of the best...
Hostage can only be shot by special forces (be a bit easy otherwise). They have to stand as directed by the robbers. Each time they are hit by a BB they randomly pull a special forces person to join the 'robbers'. So dont shoot the hostage... (Yes robbers can hide behind their hostage and still shoot back).