This is where it starts getting expensive.

Mandatory for any planned use:

• Helmet
• Small frame pump or CO2
• A tube patch kit and tire levers.
• A lightweight multi-tool for re-securing loose parts 10 miles from home.

Some bikes don't come with pedals. If yours is one of those, you need pedals.

Only buy fenders if you intend to ride in inclement weather or immediately afterward. I bought a pair that I kept on my bike for awhile, but when I left the Pacific Northwest and moved to So. Cal., I no longer needed them. Even in Portland I took them off during the summer and put them back on in the winter time.

Lights: If you will be riding at night, get a couple blinkies in back, and in front, get the nicest light you can afford. The small models with internal AA batteries are inadequate for real night riding. I recently bought a Night Rider Trail Rat 200. I would consider this the minimum requirement for front lighting.

Home shop: You need a real floor pump, and probably a few real tools too.

Gloves: For any ride longer than 30 minutes, I prefer gloves.

Bike shorts: They do make a difference.

A jersey, preferably high-visibility.

A pair of real bike shoes and clipless pedals.

A helmet liner to wick sweat away from your eyes.

A rear rack if you intend to carry a load.

A rear trunk and/or panniers.

A car rack.

.... do you see where this is going?