Maximum change for minimum dollars:
1) New set of wheels and tires. Try A23 Velocity's laced to the hub of your choice. Velomine would be a good source.
2) Change brake levers. Tektro/Cane Creek/Origin8 levers have a wider grip area that is easier on the hands, really makes the bike feel different.
3) Handlebars. My favorite bar for distance riding is the Cinelli 64-40, but lots of people also like the newer shallow drop bars like the Civia Emerson.

Change the contact points and wheels, and it will feel like a whole different bike.