Arthur's Seat is walkable from anywhere near the city centre. If you keep your brain switched on it's easy enough to get up and down without venturing onto difficult ground. Make sure yur shoes have soles that give a grip on steep grass and you'll be fine. The hill is less than 900 feet high so you could be up and down in an hour, but the whole area roundabout deserves more time. Spend a few pounds on one of the excellent guide books to Holyrood Park, and read it before you go. The park has a lot of history to it that you could easily miss.