Travel does seem obvious. I'm hoping for more specific answers, though.

He lives in Ontario, Canada. He's already been to Great Britain and France.

Agra (site of the Taj Mahal)? Giza (site of the pyramids)?

I think it would depend greatly on his particular interests. I don't think anyone can tell you, or him, what would be most memorable to him. What has he always wanted to do? Where did he dream about as a child?

Maybe I could rephrase the question: "What would be the most memorable thing YOU could do with $8000?"

The things he dreamed about as a child are likely to be petty and unmemorable given his limited exposure to the possibilities.

The most memorable thing I could do with $8000 is follow Wir Sind Helden on their summer tour through Austria and Germany, but that's something very personal to me. I've always loved the feeling associated with big, open air concerts. Wir Sind Helden is my favorite band. I'm obsessed with learning German. And I've been to Germany, and have wanted to go back ever since returning to the states. None of this, presumably, applies to your friend.

You don't need to know all the possibilities. You just need to know what you love.

I don't mean to be unhelpful but, "What's the most memorable thing you can do with $8000," sounds an awful lot like, "What's the right way to spend $8000," and there is no right way. There is no "most memorable thing." There is no "greatest fulfillment of the will." Le mieux est l'ennemi du bien. The best is the enemy of the good. There is no possible way to spend $8000 that someone won't consider awesome and someone else won't consider a complete waste.

I don't find your answer unhelpful.

Before your post, almost all of my ideas were of the form "travel to set of places X and see what you see." Now, I have ideas of the form "follow y while learning z", where y may be a band, a sports team, migratory animals, a circus, or famous trial lawyers in open court.