Iiyama's aptly-named Nanohana Park comes alive during spring, and their annual nanohana festival is held during the latter half of Golden Week -- based off previous years, plan for the 3rd to the 5th of May. The grounds are blanketed with these vibrant yellow flowers, and visitors are able to enjoy views of the Sekida Mountains at the same time – sometimes they still have a dusting of snow atop them, adding to the picturesque scenery.
The festival also typically includes stage performances and food booths for those wanting to grab a bite to eat. Admission to the park is free of charge, but bring some yen if you plan to purchase food.