Strangely, though, it seems that no one has the details here, or at least, no one has posted it, yet.
DETAILS (aka SPOILERS):
-Colossus is, as expected, the final battle that takes place on Lyoko.
-William is distrusted by the group, who think XANA may be able to retake him. This is indeed what happens (and boy am I glad! The finale just wouldn't be a finale without one final go against Xana-William.)
-Franz Hopper dies as a sacrafice so that XANA may be destroyed, which happens of course.
-No resoltion to the Yumi/Ulrich/William deal, with scenes strongly supporting either present in the last episode. DOH!
-Sissi puts a tracking bug on Ulrich and through this scheme learns the group's secret by reading Jeremy's online diary. Afraid for the safety of the world (and not knowing XANA is already dead), Sissi tells her father, who drags her away to the infirmary for a mental checkup.
-After one final return to the past, the group stops Sissi from starting her plan and invites her to be their friend instead. Sissi joyfully accepts and walks away happily and, in passing by Yumi, tells her "Hey, Girlfriend!".
-The supercomputer is shut down. The episode and series ends with this hilarious exchange (paraphrased):
Jim: What were you all doing?
Odd: Oh, just putting an end to a long and great adventure.
Jim: Ah, yes. I put away my teenage dreams too when I became an adult.
Odd: You became an adult, Jim?
Jim: Yeah...but I'd rather not talk about.
