Well, there is one that folks seem to have missed, and that is opium and LSD. Opium tends to put the user in dream state that is manageable and very vivid. When this is amplified with Acid it tends to take you to the real zanadu without undue effort and keep you there for extended periods.
Anyone who has done any amounts of acid can recall that place they go when they are just falling asleep after their trip. In that place between being awake and asleep, where you no longer have control over your thoughts but are still concious enough to realize what you are imagining - the vistas in your mind grow. Of course if you continue for only a minute or two more you fall into real sleep but many times, especialy in acid, you realize you are falling asleep, you realized how incredable your visions are and at that moment you wake up. Many times you may repeat the process but you are never able to inhabit that tiny window of peception for more than a few seconds. When you balance your intake of opium with an amount of LSD that won't simply wash the other drug away you can stay in that window for many minutes at a time.