Fair: You can jump repeatedly to get past Garble or wait for Sunfall to come and invite you to the fair. If you wait for Sunfall, you have to talk to one of the cats at the end of the fair to see Crystalblaze again.
Leader: You have to get 80 reputation and find Moss-shadow. Finishing the game is technically defeating Firestar and doing all the mini tasks after that (in my opinion), since the game doesn't have a real ending like CaliforniaClan.