The Lego Master Builder Academy kits are great ways to get into serious Lego building.
The first kit is $30, and then you can buy the next lessons in subscription packs, each of them showing how to build different kinds of things using many building techniques. It starts with the basics, but then it goes into more advanced stuff. Things that you probably never imagined could be done. And they are really just the beginning.
The thing that I enjoy most of these sets is to challenge myself in building completely different and new models with the limited set of pieces that each kit provides. Your mind will love it. The dog—Amos—you see above is my design, using pieces from kits 3 and 5. [Amazon and Lego Store]