An intricately carved wraithbone wand. Dates back tens of thousands of years, long before the Fall. It glows subtly and plays a very soft but beautiful song when it's pointed in a particular direction, roughly to the galactic south-west.
The players won't know it, but there's a beautiful tiara on the ruins of what was once a maiden world. The planet was long since overrun by Tyranids and stripped bare. The tiara doesn't do anything but look pretty. When the wand is pointed at the tiara, it starts up.
A pair of armbands. The wearer can ignore the effects of gravity for one second when jumping. It was created as a dance aid, though obviously it has adventuring and combat potential.
A carved frieze, clearly cut away from its original building in some bygone era and taken as loot by some other alien culture. It depicts a partial map of a city on a world that's since been consumed by the Eye of Terror.
A wide chalice. When pure water is consumed from it, the the drinker will fall asleep and have spiritually uplifting dreams (could be very weird and potentially life-changing for a skeptic!) When a fine eldar brandy is consumed from it, the drinker will have a vision of the future. Nothing on the chalice indicates how to use it; the players will have to discover this by accident and/or trial and error.