Take the (slightly hidden) elevator next to American eagle on the outside of Tokyu Plaza up to the 6th floor and enter straight into Omohara Forest, certainly one of the most exciting rooftop gardens in Tokyo. Not only can you get a wonderful view over Omotesando and Harajuku, but there is also plenty of space to just sit and relax (including free wifi!) without having to buy any food or drinks. I highly recommend to come here after the sun has set too, as this turns into a whimsical fairy garden at night with beautiful lights and a great view over the city.