If you want a more substantual change, consider a 12-30t nine speed cassette from Harris Cyclery. This is built from Shimano parts and will work with your current road derailluar.

See http://harriscyclery.net/itemdetails.cfm?ID=2369

Harris Cyclery Century Special:

12 - 13 - 15 - 17 - 19 - 21 - 24 - 27 - 30 Teeth
This is a Harris Cyclery exclusive, designed by Sheldon Brown, using Shimano sprockets.

This is intended to provide nice low gears and still preserve reasonably close spacing in the cruising range.

This is intended to provide nice low gears and still preserve reasonably close spacing in the cruising range, without wasting gears by devoting space to ludicrously high gears. This is intended to be a good match for bikes with traditional "full sized" chainrings.

It is generally good practice to replace the chain when you replace the cassette.

We recommend the SRAM PC-951 chain for use with this cassette.

This cassette will index with any Shimano-type 9-speed shifter and any rear derailer.

This cassette will fit any Shimano 8- or 9-speed hub.