Here's my top 10 but I could probably come up with at least 10 more
1. Avionics - you will always want more/better
2. Gizmos - Headsets, flashlights, handheld radio, iPad
3. Subscriptions - Garmin Pilot, Foreflight, etc.
4. Taxes -
5. Training - You'll fly more which means you need your IFR...