You might want to think about an 11-34 rear cassette. This will require a rear derailleur change, but it will give you some nice gearing. The Shimano Deore RD works well and is not expensive. I've set up several bikes this way for friends and family.

This is the way my commuter bike is set up, and I genetrally use it for most rides rather than my "good" road bike. I also run smaller chinrings on it.