Arthur's Seat is an ancient volcano located in Edinburgh and is the main peak of a group of hills that form Holyrood Park. It stands 251m tall and provides excellent panoramic views of the city. It can be climbed from any direction but the easiest route is from the east, with a grassy slope leading up to it.
The walk does take longer than many expect and is steep in places so make sure you have the right footwear / clothing and give yourselves time to get back down in daylight.
Some history:
A hill fort is believed to have occupied the summit in prehistoric times and is said to even have served as the legendary Camelot. There is also evidence that the Votadini, an ancient British tribe, had settled there. Arthur's Seat is now a popular spot for streak-walking, rock climbing, and taking in the sights and sounds of Edinburgh.