Laser Quest is a good cheap alternative to paintballing, cos by the gods, the costs there add up quickly. Even more fun if you smuggle bottles of beer in. There's one place up in Cleethorpes that is the most low-rent, scabby place you can imagine - the lasers look like converted Bosch power drills - but it's full of bouncy castles and ball pools and slides as well, it is brilliant.
Could always go for a long hike somewhere round Wales or the Peak District if you're so inclined? Or hire a canal boat, then some of you can get drunk and your sober friends can steer ;)