1) Kenner Aliens Toys
And I have to include their infamous 1979 18" Alien Doll, which I saved and saved for as a kid until I could finally buy one with its box (yes, I had to have the damn box too).
2) Monster in My Pocket
3) Ghostbusters Monsters
These were the most amazing things to me as a kid (ok, besides Cameron's masterpiece Aliens). The mummy toy always reminded me of Monster Squad because you could take off his arm and head with only his bandages holding them on like when the mummy came unwrapped in the film. If you're wondering (like I was) why the heck they called the cartoon (and these toys) "The Real" Ghostbusters, Wikipedia says it was added to the title after a dispute with Filmation and its Ghost Busters properties. Still seems stupid to me since they're obviously the same with the same characters, same song, etc. and I never even noticed "The Real" there before.
4) Robocop vs. Terminator (Genesis)
5) Alien vs. Predator (SNES)
My brother and I only had a Sega Genesis as a kid so I had to play SNES over at a friend's house. Of course, if I had my own Monster Squad tree house, I'd have both. This was another game I loved to death. Basically, just Streets of Rage but you get to be Predator and beat the shit out of Aliens (and pick up skulls and food along with a ton of his signature weapons like his spear, throwing disc, etc.). The cut scenes were great for the time, and I thought the music and sound FX were excellent. Writing about this reminded me of F-Zero and Contra III, two other incredible SNES games.
6) Fangoria and Cinefantastique
7) Cinefex and Famous Monsters of Filmland
Yep, I couldn't leave these out. If I had all these magazines, I'd never leave the tree house, but that'd be fine by me.
8) Halloween Masks
I remember always seeing a Predator mask up at Spencer's in the mall. I wanted that more than anything, but I never had enough to buy it (I was probably 10). It was pretty expensive, and now it's almost impossible to find because all you've got are crappy AvP masks, which I couldn't care less about. Of course, I'd love to have an Alien mask as well or the full freakin costume, but I'm dreaming.
9) Stephen King's IT
10) Godzilla Posters