I've been to Cheonggyecheon a number of times now, and it really never gets old. There always seems to be something going on, and it's a pleasant spot to take a nice leisurely stroll. When I visited this past Saturday while on a short trip to Seoul, there were a number of decorations on display to help celebrate Buddha's birthday.
If you're visiting Seoul for the first time, there's a kind of historical/modern circuit you can do. I like to start off at Gyeongbokgung, which is the largest of the Five Grand Palaces. You can easily spend a couple hours immersing yourself and fully exploring the grounds. Your ticket to the palace includes admission at the National Folk Museum which is on the grounds there if you're interested. After you're done, walk through Gwanghwamun Plaza, making sure to take a quick stop to see The Story of King Sejong Exhibition Hall underground, and then on to Cheonggyecheon to walk as far as you please.