From the Washington Post Style Invitation

SWISS ARMY KNIFE - das Messer - should be masculine, because even though it appears useful for a wide variety of work, it spends most of its time just opening bottles.

KIDNEYS - die Nieren -- should remain feminine/female, because they always go to the bathroom in pairs.

TIRE - das Rad - should change to masculine/male, because it goes bald and often is over-inflated.

HOT AIR BALLOON - der Luftballoon - should remain masculine/male, because to get it to go anywhere you have to light a fire under it ... and, of course, there's the hot air part.

SHOE - der Schuh - should remain masculine/male, because it is usually unpolished

HAMMER - der Hammer - should remain masculine/male, because it hasn't evolved much over the last 5,000 years, but it's handy to have around.