Short answer: pick up a copy of Ernie Zielinski's "How to Retire Happy, Wild, and Free" at the library and do a "Get a Life tree".
Long answer:
learn a foreign language, or several - this helps with your travels
learn how to play an instrument
take up a physical challenge
take up a hobby that creates things, make gifts for others
volunteer to help others
read - philosophy, history, biographies, fiction...
take the college classes you couldn't fit into your schedule the first time around
Head over to early-retirement.org and read the entire "What Did You Do Today" thread for inspiration.