Dozens of spring perennials are in bloom right now, but none are revered so much in Japan as sakura, or cherry blossoms. The pale pink blossoms hail the true arrival of spring, and their brevity (the shower of petals lasts about a week only) has symbolized the fragility of life for centuries.
This doesn't mean you're about to read a treatise on the beauty of sakura. There's a bacchanalian aspect of cherry blossom culture that's just as fascinating as the highbrow one, and even offers insight as to why sakura picnics in city parks are so rowdy. To escape those, try finding some quiet expanses of yamazakura (mountain cherry) to admire. Kyushu's highland sakura bloom after mid-April, and make for fabulous post-hanami peace.
I'd once thought hanami (cherry-blossom picnics) would be civilized gatherings of gentle folk sitting under trees writing poetry while uttering blissful "oohs" and "aahs." But the nearest I've come to such ceremony was hearing a drunken coworker bellow "wabi-sabi!!" at the top of his lungs -- referring, supposedly, to the Japanese concept of subtlety in all artistic things. People all around roared songs into portable karaoke kits and gulped down truckloads of alcohol. That's been my image of hanami ever since.