Every month, I allot $20 or so in my budget to purchase FSX addons. My latest one, the Carenado C340, has been one of the best purchases I've made on an addon. Fun airplane to fly; the rudimentary AP makes it, as I know it, difficult to simply push the buttons and go nap.
I usually like to challenge myself in difficult flying conditions. In addition, as I'm currently in ground school, working towards getting my (real) flight training, I like to practice NDBs and VORs on the sim, as well as to hone in my airspaces knowledge.
If you are looking for some fun, you can go fly in one of those glitch airports in FSX. I lived in Sentani, Papua for 12 years, and the airport there (WAJJ) happens to be one of those glitch airports in FS (ever since FS2002). In real life, the Rwy30 end of the airport meets a lake, but in FS, this lake is 500 ft higher in elevation than the airport - WAJJ in FSX is effectively in a huge bowl of land, surrounded partially by a lake. Now try flying the Rwy30 approach in a 747; (1) the airport is only able to accomodate a 737 in real life, and (2) you'll have a 20* approach angle!