If you're a rabid Hello Kitty fan, welcome to kitty heaven @ Siam Square One, which is just beside Siam BTS station. Walk through the mall and it's on the other side.
You can order something to go from their side booth beside the main entrance at the street level (soft serve ice cream 80 Baht; milk tea/iced tea 70-100 Baht) but who wouldn't want to walk in and take a peek?!
On the first floor (@ street level)
The merchandise where little girls go crazy are sold at the basement. It even has a Hello Kitty car parked inside.