I would agree about the voltage tester, the claw hammer or at least a heavier hammer, and the small level; I use a caravan level, but even one of the small set squares with a level in it is quite useful.
Have both posidrive and philips screwdrivers.
I would have though a bright (but cheap) head torch may well be absolutely invaluable.
A battery screwdriver/drill will be useful fairly soon.
I applaud the initiative in getting the cup of tea for the boss ready early, but it may get cold by tomorrow!
I hope it all goes well and always remember that you have to know something before you can get it right so ask first.
Hopefully your first day may be a bit of induction, but sometimes it is right to it!
Very best of luck.