Motorbikes, motorbike engines, motorbike models, famous motorbikes and motorcyclists, including off road bikes. Cars, car engines, car models, famous cars and drivers be they: F1, off road, rally cars, touring cars etc etc..
Also:
Guns, rifles, weaponry;
Air conditioning units;
Engines and aaaaallllll the different types there are
WWI, WWII, Vietnam War, Falklands War, Bosnian war, Gulf War (all incarnations) just war in general really, particularly tanks and armoured vehicles (back to cars and engines again aren't we).
Good grief, even with an interest in history, I'm not sure which subject is the most tedious after 15 years together, I should be an expert from hearing it over and over but I'm not.