My two oldest boys are three years apart but not much different in size and share a lot of the same friends, plus their birthdays are 3 weeks apart, so we do their parties together. For the party that they were 7 and 4 we had an FBI theme.  My husband is a special agent so we got a lot of free "stuff" from the office - bumper stickers, pencils etc plus bag. My husband went to the dollar store and found handcuffs, disguise kits, toy guns (sorry for those of you who don't approve but when daddy carries a gun for a living, guns are a part of our lives) etc. Then he wrote up a scavenger scenario.  He taped it as if he was the FBI director but mission impossible style -even with the music.  When the boys and their buddies (there were only 4 in all and it worked well, more would be fun too) were eating cake I ran around the house and placed the tape at the door and rang the bell.  One of the boys answered it and there was the tape the cat happened to be there too so that was fun - they thought the cat brought the tape.  They listened and dad wrote down the directions, then they went on their hunt.  They found their things all for the end purpose of finding the bad guy who just happened to be DAD!!! In the end they all jumped on dad to keep him from getting away.  Since it was a surprise that it was FBI themed we did an average cake but you could add to it with special supplies.  It was a lot of fun.