The City of Seattle has built a very nice "Education Center" at the south end of the recreational area. While the displays are not all completed yet, it is still quite worth seeing. For further details and when they are open, and other interesting information, see the Cedar River Watershed Education Center website. Or call them at (Seattle number) 206-733-9421, or (North Bend) 425-831-6780.
During the summer the Education Center is open (speaking generally and non-authoritatively!) Tuesdays through Sundays, 10 AM to 5 PM. Even if the Education Center itself is closed, it is a very interesting, and very restful, place to visit. If your hike gets rained out, go over and admire the grounds (from the covered walkways!), listen to the rain drums, and relax. However, do note that the access road gets locked at 6 PM.
(By the way: Seattle Public Utilities recently redesigned their web pages. And broke all existing published web links to the Watershed, and forgot to add new links. And buried their web pages so deep that neither Google nor the City of Seattle's own search service can find them.)