Giant Monster Rampage
Giant Monster Rampage is essentially a rule book for playing with your toys. Everything from the figures to the terrain is either a figure you already have or a repurposed bit like a book covered with a piece of felt to be a mountain. Buildings can be LEGO bricks or another compatible style of blocks. The only thing you might not have around is the ten-sided die to use for your rolls.
With the character creation toolset of powers and abilities, you can make just about any monster you have a figure for. Most of the powers are vague enough so you can just rename it and have it be the one you are trying to recreate.
The game is easy enough to pick up for a standard game. I played it with my daughter for a few games and after a bit we didn't need the rulebook for the basics and only referenced it when we had a question we didn't know the answer to. We did a one-on-one battle first to get the hang of it and then branched out into city destruction and a "destroy all monsters" scenario and then went rather well.
My only complaint is that after a few games with makeshift terrain I started looking online for Heroscape terrain and Monsterpocalypse buildings to use to give the maps a bit more character and I have a feeling that it might be an expensive game in the long run because of that. Otherwise, it has a low investment ceiling for someone who already has 3" or 6" figures like I do, which is nice as buying more toys isn't something that I really need to do.
It's a fun game for a decent price and even if you only play it a few times with your toys, the book isn't going to take up an extreme amount of space like some boardgames might. Definitely worth a look if you are a giant monster aficionado like myself.