Pretty much everything.
If i can give you one tip tho.
Something I wish I had done.
Set up a folder called Templates on your desktop.
Inside create 2 new folders.
one called original.
one called custom
before you amend a template in admincp. select the code, copy and paste into notepad, and save as example: forumhome.txt into your original folder. Then do code changes in admincp, and after clicking save and reload, select the code, copy and paste into notepad, save as example: forumhome.txt into your custom folder.
If you are really really good, then open a new notepad thing, and diarise each edit or at the very least the dates you do the edits, and also dates you add modifications/plugins and the like.
Really will make life a lot easier mate