Flexibility, Core Strength and Leg Strength. These are areas that every serious cyclist should be looking to improve in the off-season. So what will each one do for you:
1. Flexibility - increase your power output - especially when TT'ing, reduce the risk of injury.
2. Core Strength - increase your power output, improve bike handling.
3. Leg Strength - increase your power output.
As you can see, increasing power output is common to improving in all these areas and isn't that what we all want. Three is a no brainer except for maybe the most elite level of cyclists. Most people know about #2, but may not do anything about it. With #1, most people understand the importance of stretching, but may not understand that lack of flexibility will hamper our muscles ability to operate correctly and thus, produce less power.
Anyway, this is what my athletes will be working on for the last few months of the year while still getting in their share of miles. Here are a few links for core exercises and other off bike work:
http://training.active.com/TransactionManager/viewPlan.do;jsessionid=537108FCFD7AEFC4E69B431A9F3F8C25?tpId=146 (be sure to click on the "strength training" days to bring up more detail)
http://link.brightcove.com/services/link/bcpid1138272038/bctid958475643 (from Fit Chick at Bicycling Magazine)
http://www.coreperformance.com/eodArchive.php?p=1&s=3 (Exercises of the Day from Core Performance)
These three sources should give you plenty of exercises to choose from to constantly challenge your body to adapt.