Outside the building are great ways to relax and enjoy great weather. There is a park area between the two towers which has some nice features to explore.
If you enjoy parks how about exploring the Shinjuku Chuo Park behind it:
On the 45th floor you can have a magnificent view of the area around you and see the Sky Tree Tower from afar:
Inside the building on the 45th floor, there is a two part toy park. You can buy anime goods but also character goods. Anime goods include Pokemon, Detective Conan, My Neighbor Totoro and many more. But there's not only japanese animation goods. You can also find Disney merchandise there. The merchandise comes in a variety of goods such as mugs, mini and 3D puzzles (see photo below), clear files, key chains and stationeries. They also have regional goods available which you cannot find anywhere else!
Other than anime and character goods, you can also find the usual japanese souvenirs (e.g. key chains, magnets, elongated coins, etc.), capsule toys (anime and non-anime), merchandise for the Tokyo Olympics held in 2020 and a restaurant. If you're lucky you can also see wedding ceremonies there.
If you collect commemorative stamps which are available at many sights and museums, I advise you to bring some paper or your stamp notebook as they don't provide any. They have a new stamp this year (the old one is also available), so if you enjoy collecting stamps and only have the old one you might want to consider revisiting. ;)
Inside the observation floor is not much space to sit down so after taking a good look around leave the building and go to the outside. There you can find beautiful sculptures and seasonal flowers.
To promote peace Tokyo citizens created "An Appeal for Peace by the Citizens of Tokyo" close to the statue above:
You can also have a different view of the observation building from this place!