Watch the Credits
If you go into Desmond's room after decoding the symbols in the main part of the lab, you can trigger the game's ending credits. Walk up to the foot of the bed and press the button you would press to use Altair's Eagle Vision while in the Animus -- "Triangle" for PS3 and "Y" for Xbox 360. When you look at the wall behind the bed using Eagle Vision, Desmond speaks and the credits roll. When they are finished, you regain control of Desmond.
Use the Animus
When the main storyline is over, you can get into the Animus and replay any of Altair's missions. Just walk up to it and press the button you would use to get into the Animus during any other part of the game -- "X" for PS3 and "A" for Xbox 360. Use the virtual interface to select a mission, which will bring you to the loading screen. You can use this opportunity to find the rest of the flags in the towns or to complete more side missions.
Use Lucy's Computer
When you regain control of Desmond after the credits roll, you can access Lucy's email account as you did earlier in the game. Many of the messages are the same ones from earlier, but if you go into her deleted messages, there are some new ones. These emails are, presumably, from the assassins that tried to rescue you at the end of the game. To decipher them, use all of the capitalized letters to form a secret message.
Use Vidic's Computer
This only works if you picked Vidic's pocket after completing Memory Block 4 when you killed Majd Addin and read Lucy's emails. You can pick his pocket first thing in the morning before starting Memory Block 5, Memory Block 6 and Memory Block 7 -- when you do, you get his pen with his computer's password. Read his emails to learn more about Abstergo, the Templars and the Animus and its side effects.
Get into the Conference Room
The conference room is to the left of Desmond's bedroom and has another computer terminal you can access after watching the game's credits. You must have stolen Vidic's pen and read Lucy's email to access the conference room and its computer terminal. Read the emails on the computer to learn more about Abstergo.