I picked up a relatively inexpensive trunk rack, with the velcro straps to hold it onto the top of the rack, from REI during one of their sales. Works quite well for simple trips around town. Holds water bottle, roadside repair kit (including tools, patches, and new tube), a change of clothes, and all my electronics/wallet/etc.

For shopping and commute to college, I also picked up an inexpensive set of paniers. I've loaded them rather full with books and clothes, and still had 15lbs to spare on the max rating for the rack.

The trunk bag has a rain cover attached, and I carry plastic garbage bags in the paniers in case rain falls (a tip I picked up here a few years ago).