To go and look at interesting things, especially in a place you are visiting.
Sightseeing is a fun and interesting thing to do when you are in a new place.
Examples:
We will sightsee in the city today.
She likes to sightsee in new places.
They sightsee at the big tower.
I don't want to sightsee in the rain!
When I sightsee, I see all the things!
We sightsee at the zoo and see all the animals.