Reed,
An extension cord shouldn't be a problem, since it only consists of copper conductors and insulation, and will work just the same on 220 VAC as it does on 115 VAC. The main concern with the higher voltage is the insulation ratings, but at those voltage levels that shouldn't be a problem. An extension cord will not contribute to "frying" your batteries, as it's simply (as the name implies) an extension of the power outlet.
A simple Power Bar with NO surge protection or other electronic circuits should also work fine. THIS is one model that is configured for travel use on the higher voltage.
As Frank mentioned, you'll need to check EACH device that you plan on taking to Europe to ensure that it's rated for operation from 100-240 VAC. Look for the wording Input Voltage on the charger or on the device to see the ratings.
You will of course also need Plug Adaptors specific to the countries you'll be visiting.