Hello and welcome to the forum!
There are so many things you can do the options are almost unlimited. I suggest you first press on with doing the examples in the "Getting Started" book you mentioned. Ideas may spring to mind from that.
I would formulate a clear idea (not necessarily today) about what you want to do, then the appropriate purchase to achieve that may be clearer. Do you need wheels, tracks, hands, sensors, radio control, battery power, etc.?
Some pretty cool projects are made with minimal parts (TV Begone for example). Others involve music (eg. MIDI), graphics (eg. LEDs), motors, controllers and so on. Maybe you have a cat. Maybe you want to automatically feed it when you are away for the day. The exact things you need to buy depend on what you really want to achieve.