I think I've got all the essentials I need for tooling around town on my new Trek.
I've got my helmet; an insulated water bottle & cage; and a wire basket & rack on the back for library books, groceries, or whatever else I have to haul around. I've got front and rear lights too.
For longer rides, I've also got a bike computer, a bicycle pump, spare inner tubes, adjustable wrench, and allen wrench set. (Perhaps I'll get a patch kit after I learn how to patch flats.)
After I lost a check from my pocket that I was riding to the bank to deposit, I bought a small handlebar bag to safely hold my keys, money/debit card, phone, papers, etc.
With the essentials taken care of it was time for decorating, so I added red, white & blue streamers.