A Mountain Myrricle mirror, so you will be more aware of your surroundings.
If you decide to change grips, or add bar ends - I have some favorites. I have both Ergon and Specialized ergonomic grips. I like the Specialized ones better, as they are slightly softer, and just a little more comfortable. I also now prefer the Specialized P2 Bar Endz, to the tubular ones (I have both)
Those would be my first suggestions (after finding the seat you like best). So, come back with questions about how to fix problems you find, or how better to make it your own.
Other than those - there's lots of stuff I have changed over the years, to fit my bikes to me.
Now, go get some miles, and enjoy your new bike