That's a tricky question actually...I can't answer you fully but I would suggest some cool gift shops like @Hamley's if it's a kid gift or @Bath & Body Works for women products

Where can I buy all things with Hello Kitty on it?

@Hamley's I guess

Where can I find a large selection of Burago model cars for a good price?

Have you tried @Hamley's

Where can I find a witch costume?

@Hamley's has a decent variety of costumes as well!

Where can I buy blow up swimming gear for kids like floaters and rafts?

Try @Hamley's. Good luck!

Where can I find a darth vader toy or figure or merchandise?

@Hamley's - They should have Darth Vader toy figures.

Where can I buy items for a masquerade party?

@Hamley'ss Mecca Street

Where can I buy toys for my nephew and niece?

@Hamley'ss Mecca Street

Where can I find a place to buy Action Figures?

@Hamley's

Where can I buy 3D puzzles?

I saw a few 3D puzzles on the second floor of Hamleys on Mekka St. Hamleys is just past Applebees and Fudruckers.

Where can I buy Gift for kids?

@Hamley'ss Mecca Street - 3 Floor of amazing toys :D

Where can I Buy a cowboy hat?

@Hamley's - Costume section

Where can I buy a small musical keyboard that's reasonably priced?

@Hamleys