What to do after credits on assassins creed

absolutely nothing, you have to go back to your last save which was were desmond waked up for the final time, but you can still explore by going into the animus and selecting what part you wish to go to

Edit: You can go to the table Lucy works on and read the mesages from her and vidics emails and then go into the room where you can see through a glass. With the key you can type the password and go into that room where there is another laptop allowing you to read an anonymus guys emails. That's as far as I got.