Leif brings the sun
Leif will appear in your town at some point during the two-week long Nature Day celebrations - he will likely appear multiple times, actually.
Leif will come supplying flowers not native to your island, and new bushes, such as the Azalea bushes and the Holly bushes. These can be placed and planted like trees, but don't require as much space, and will bloom into a small, sweet, bush.
They're purely decorative, but if you plant 20 you'll find yourself earning a whole bunch of Nook Miles, and clearing some new challenges.
Building the hedge fence
Oh, but that isn't all of the challenges included in the Nature Day update. You will also have one unique Nook Miles+ challenge each day pertaining to the Nature Day event, and you need to clear at least the first one.
Once you've cleared one of the Nature Day Nook Miles+ challenges, speak with Tom Nook. After some exposition, he will give you the DIY recipe for the hedge fence, which is placed in the same way as any other fence.
It's quite nice, and should make your residential districts and flower gardens look very striking indeed! Once you have the recipe, you can craft it with stone, branches, and weeds.
Crazy Redd and the Museum upgrade
Along with the update, Crazy Redd, the slightly malicious kitsune, will appear in your town. It might take a day or two.
Once he appears his ship will be docked at the secret beach, and you'll find him walking around town. He'll offer you a piece of art, at a high price.
Buy it off him, and then donate it to Blathers. This will trigger the museum upgrade, and the Museum will be closed for renovations the next day.
May day May day
This isn't available quite yet - unless you're sneaky and time travel (don't do it) but early May will bring with it a whole bunch of brand new activities to participate in.
These include May Day tickets, which will take you to special islands where you can find unique items, flowers, bugs, fish, tools, and more. These islands will play out like challenges or puzzles using the Animal Crossing mechanics you're familiar with.
These look very exciting, and when paired with all of the other content that has been added to the game in its first month, the future of Animal Crossing: New Horizons looks very bright. Now let's hope COVID doesn't delay anything…