Congrats on the bike! 20 miles a week isn't much though ... if you catch the bug you'll be doing a whole lot more than that soon enough.
I'm having to downsize my own purchase a bit (105 components instead of ultegra) just to help cover some of the extras that seem to come up.
My list (I have a bunch of stuff already):
tube/patch kit/co2
floor pump
bibs, jerseys & rain jacket
another bike computer (giving the garmin 500 to the wife, looking at 705)
sunglasses
bottle cages
new helmet