My daughter will sit and play with my old Lego for hours, building all manner of things, from houses to cars, without any prompting from me. But then she is also quite happy to get covered in oil helping me fix my Land Rover, so maybe the environment she grew up in was against her :-)
She is, however, definitely not a "tomboy", as she has the usual fascination for all the pink tat that is nowadays thrown at young girls, and the usual clothes, makeup etc that is "normal" for a girl of her age.
On another note, when I was young I used to have some lego-like construction bricks which belonged to my dad, which were made of a type of rubber, rather than hard plastic. Anyone else have anything similar?