USA Today has a list of tips for American tourists who don't want to get hassled by Europeans disgusted with the Bush administrations push for war.
Avoid American fast-food restaurants and chains.
Keep discussions of politics to private places, not rowdy bars.
Take a rain check on wearing clothes featuring American flags or sports team logos.
Keep your passport out of sight.
Keep cameras, video equipment and maps tucked away.
Soften your speech; Americans typically overshadow their hosts in the volume department.