+1 on pump as item #1
A multi tool is also an essential. If you don't already have one get one. (Unless you want to carry separate tools) A seat bag or some other form of holding device would be very high. (You need somewhere to store the extra tube(s) and/or patches as well)
For the tail light I just got the Radbot 1000 and it is awesome. I WILL BE NOTICED!!!