Gonna agree with Princess Mononoke, Paprika, The Animatrix, and FLCL.
Other Miyazaki movies (Spirited Away, Howl's Moving Castle, Ponyo, Totoro) are also superb and, frankly, friendlier and more colorful than Princess Mononoke. Other Studio Ghbili contributions are still great, but especially Pom Poko. The only movie of their catalog that I would not recommend while tripping is Grave of the Fireflies. Memories (Magnetic Rose), is also a darker anime but it has three shorts and 2 of them are funny/weird enough that they would be enjoyable while tripping. I can also recommend the Trigun series and movie and Cowboy Bebop, as well as the recently released series Space Dandy. The Triplets of Belleville is great (although a departure from Japanese animation). Weird but definitely meant for people on drugs would be Adventure Time.
Pixar movies should not be overlooked either (if computer animation is fine), especially Up, Wall-e, and Finding Nemo.