The way I have run this for 7/8yos is:
gaggle of boys arrive (doesn't matter if it's on the day, usually on the nearest weekend to birthday)
greeted by dh who does the honours (I am skiving busy in the kitchen)
handed first clue in a treasure hunt which they are told they must do as a team so noone gets left behind (or the treasure will mysteriously vanish)
treasure hunt is linked to theme of party (last year we did pirate's with a map of the house as Treasure Island)
they are sent up and down the stairs and into the garden until they find the treasure which on the last two occasions has been water pistols (bearing in mind that ds's birthday is in May)
wild water pistol fight all over garden, with dh refereeing (and getting soaked to the skin)
food (a few sandwiches, but mainly crisps and biscuits- they never eat much at parties)
cake
party bag (we let them fish for theirs with a fishing rod, blue cloth stretched over living room door frame represents the sea)
when they were younger we did more sedate games instead of the water pistols: things like Musical Bumps or Sleeping Lions