I'm not into the figurine parts of legos, but I absolutely LOVE the freaking building and designing part of it.
Give me a whole crate(s) of generic legos, an hour, and I'll build a whole city.
a couple years ago I used to mix legos with thomas the train shit and make complex train patterns, because with legos I can make the tracks go much higher (like a good foot upwards) and all that. The trains are oddly powerful enough to make it up the tracks. (I made this stuff for my 4-5 year old little brother. He used to pester the shit out of my to make a new one every day).