A stamp set featuring the highest peaks of each continent excluding Antarctica:

North America - Denali

South America - Aconcagua

Europe - Elbrus

Africa - Kilimanjaro

Asia - Everest

Oceania - Puncak Jaya

I actually got the idea from another hobby, Geocaching, where you could earn 'souvenirs', something like achievement badges, featuring these mountains as part of their "Reach the Peak" challenge last year.

The design should be different of course, but just to give you an idea, this is how they looked like:

I hope this doesn't count as "stealing" the idea - I was thinking about something that could be featured from each continent and this feels like a good option. Thanks to that challenge, I learned about mountains I hadn't heard of before so I think it would be nice to spread the knowledge here, too :)

A similar idea could be the longest rivers or the largest lakes of each continent as well.