You could go to a park and have a scavenger hunt. This takes bit of organising, someone has to go there early to set up the clues. The boys can be broken into teams and with each team getting a colour and they have to find the clues to get to the next point, but make the clues in a different order for each team.

Outdoor playgrounds are good because you have the options for a lot of ball sports too.... or even make some newspaper kites and have them fly them. A lot of $2 shops have kite string and holders cheap and then you just save old newspapers, pre make the cross sections with some cheap thing garden stakes and get the boys to finish the frames off with the news print... old shopping bags make good ribbons for the tales, and its re-cycling...then they can fly their kites in the park. Instead of lolly bags they get a kite to take home.

Again at a park you could do his party as a mini olympics... set up a little race track and have egg and spoon races, or an obsticle course, you could have a long jump... then with little bags filled with sand and a target painted on an old sheet they could toss them to see who gets the closest. You could even make limbo and event... and you could have made little medals for each event.