Let's see, 3 months...I haven't memorized everything that happened, gimme a sec to look up all the important highlights.
Well, you missed Nemesis Draft--a draft mode where, after banning, you pick the champions on the enemy team, which resulted in a lot of Urgots and supports. Also, URF mode came back and then just now left.
You also missed out on AS Bruiser Fizz; for a few patches, Riot made his W deal the %HP instantly on the active portion, instead of as a DoT on the passive portion. But they've reverted that, because AS Bruiser Fizz was a bad thing.
Tristana was reworked. Mostly, her E was changed from a DoT to a bomb, and it lost Grievous Wounds.
DJ Sona ultimate skin came out. A nice, 3-in-1 skin that you can switch mid-game.
Kassadin was changed from long-range Riftwalks to short-range Rifthops with a lower base cooldown.
Veigar got reworked, and right now all he really needs is for his W to be more reliable for it to be an acceptable rework.
Zilean got reworked, and he's pretty cool now that he has a stun.
Revive was removed.
Bard, new champion, a support based on wandering. People are still having trouble working him into team composition, because of how his kit is designed. It kind of lacks any real combos, and he has almost no personal power. Also he has an AoE Zhonya's for an ult, which affects literally any unit.
An intermediary item for Sunfire Cape called Bami's Cinder was made, and it also builds into a new jungler item enchantment called Cinderhulk. All of their fire AoEs do not stack with each other. Oh, and Cinderhulk brings back the old +25% bonus HP item.
AP champions have their own version of Statikk Shiv; Luden's Echo is a high AP item with a bit of MS. Honestly, I'd trade some of the AP and/or the MS for CDR, but it's pretty nice nonetheless. Adds a bit of burst and AoE.
Champion Mastery, a system to track how much (and, in theory, how well) a person plays a specific champion, is being worked into the system. It's not fully implemented on Live servers, but it should be coming Soon™.
Because of Cinderhulk, the tank meta is eclipsing the assassin meta, and Sejuani is considered god tier.
Aside from that, a plethora of buffs, nerfs, tweaks, texture rebalances and other changes have taken place over 3 months. I listed all of the ones that were at least somewhat high-profile. If you need to know changes on specific champions, you could review their individual patch history through the LoL wiki.
Also, welcome back.