A t-shirt in Southeast Asia that you have seen on at least 10 other backpackers (the one with the beer brand, or the ‘Same Same but Different’ text).
Souvenir-tip: The big souvenirs may look funny or cool when you see them in the country of origin, but if you don’t have already a designated place for them in your house when you decide to buy them, they will just be collecting dust until you throw them away. What I do is usually just get a small souvenir from the country where I’ve been, and put it on my bed side table. That way I can see it every day, have the memories of it, and not clutter my house (or the attic) with unnecessary junk.