Basically anything you set up for the purpose can function as an altar.
Particularly in India, the cosmic significance of the altar was fully explored. The ancient sages saw it's different parts as representing the various sections of the universe and concluded that its construction was a repetition of creation.
Certain items are often used to define the space and function of an altar, others for
focusing energy and create space, and invocation of higher vibrational spaces and beings.
One of the most important conditions for accessing altered states of consciousness is the switching from beta brain waves predominance to other brain-waves states. This can be induced with the help of sounds, smells, moods, meditation, relaxation techniques and through one of the most potent tools I have encountered: the Beacon and the Superbeacon.
Items like these are often used on an altar:
All these aim to aid in the accessing of different vibrational spaces, creating an atmosphere where the supernatural, other dimensions and altered states of consciousness become accessible.
An altar also often holds pictures of spiritual leaders and teachers, flowers, statues, fabrics, crystals, gemstones, feathers, bowls of water and rice or sand. Anything you hold precious and honor as divine is an appropriate item to put on your altar.
It is not necessary to take drugs to have an experience of breakdown of barriers to other realities. Altered states experiences can be accessed with practice.
It serves one well to have some experience in grounding and getting back to consensus beta brain reality :). Sometimes it takes a bit of adjustment....