Gifts of Mani: Sleep. Good dreams. Prophetic dreams. Healing. Blessings on travelers. Protection of children and the mentally ill. 
Symbols: obviously anything shaped like the moon or with the moon on it, hour glasses, old watch parts, knots, time pieces, calendars, mathematical equations, musical scores, beaded necklaces, mirrors.
Colors: blues, silver, black, purple/lavender, pale white.
Rune: Dagaz (I also happen to see Ehwaz with Him)
Stones: moonstone (obviously), labradorite, selenite, quartz, amethyst.
Food and drink: Sambuca, cookies (especially ones with marshmallow or odd shapes), angel food cake, peppermint flavored sweets.
Other Offerings: jasmine, carnations, night-blooming flowers, highly aromatic flowers, mugwort, blue sea glass, any volunteer work or donations that benefit abused children or the mentally ill.
Things not to do: harm or abuse a child in any way; mock the mentally ill.
Organizations to donate to in order to make an offering to Mani:
International Society for Prevention of Child Abuse and Neglect
Child Abuse Prevention Organization
National Alliance on Mental Illness
American Sleep Apnea Association
(Artwork from the Giants' Tarot by HaloQuin)