Tons of excellent note taking and productivity apps: Noteability, GoodNotes, PDFExpert, Dropbox, LiquidText, Google Docs and Sheets, Numbers, Keynote, Duet... just to name a few that I use.
Lots of decent image manipulation apps. Go get all of the Adobe apps.
Kindle and iBooks are both great on the 12.9" iPP. I just finished reading my first book on it and I love it. I shrunk the default text size in iBooks a couple of notches so TONS of text fits on each page (meaning you have to turn pages less). I use the Kindle app for textbooks and it is PERFECT on the huge screen.
Social apps are good: Facebook Messenger, Google Hangouts, Slack, etc. Facebook still needs an update for the bigger screen but the app works fine as it is.
I also think the Apple News app is great on the iPP. Feels like reading a nice magazine.
The new game "INKS" is pretty cool on the big screen... check it out!