And I'm really enjoying this game again! I think the bell at the start of the round is a great feature, and all these new items look really neat. I quit about 3 years ago and I'm wondering if there are any other cool features that I might not know about.

Glad you're back.
:-D

Welcome back!

Yes, a lot of things have been added/changed while you were gone, some of them may be interesting and others may be disappointing:

The event system was changed. Instead of having events on special occasions (Christmas, Halloween, Easter, Valentine, etc), now they're adding events every two weeks. At the beginning it was only every (1) week, but they changed it to two because a lot of people couldn't get online to complete the event, so two weeks would be a bit longer and people would have enough time to complete the event.

-Two skill trees were added.

-When you get cheese, then you die, and get revived, you get revived with cheese.

-A new command (/time) was added, it shows the time you were connected to the game in (Days, hours, minutes, seconds).

-A new coin system was added, now you're able to collect coins when you save mice as a Shaman, in Survivor, in Racing and Bootcamp.

Those are just some of the changes to the game, there's a lot more:

You can check the Announcements section of the forum to see the new features/events/other announcements added to the game.
You can also check the Small Announcements where they announce some of them announcements, such as bug fixes, features added to the game and forums, etc.

There's also General 2016 Adventure Guides and Discussions, it's always updated with new information about the events added, and there will be guides. You can also share your opinion about the events added and ask event-related questions and discuss about them.