Capture the daffodil and tulip fields in Mt. Vernon. This is an annual event every spring. Combine your day with wine tasting at one of the several wineries in the area.
Skagit Valley is an important agricultural region of Washington State, but the prettiest time of year to attend is during the spring when flower season takes hold. Daffodils first, then tulips, and then irises bloom and cover fields as far as the eye can see.
The Skagit Tulip Festival is held every spring from April 1-30, but the bloom is dependent on mother nature's whims, so you might be able to visit outside this time as well. During the festival there are a lot of scheduled events to partake in, but it's often nicer just to head up there with few plans other than meandering through the part of town where flowers are grown, stopping every so often to take photos or popping in to a winery for some local tastings.
If you're able, go early in the morning or later in the afternoon/early evening on the weekend to avoid crowds - better yet if you can visit on a weekday. Bring your bike for more freedom and less parking hassle! Most establishments in the area will have free maps you can take that will show you where each flower field is. Enjoy just chilling, photographing, and enjoying the awesome colors!