Random Magic Ring
A ring of spell storing contains up to ten levels of spells that the wearer can cast. Each spell has a Caster Level: equal to the minimum level needed to cast that spell. As with a wand, the user need not provide any material components focus, or pay an XP cost to cast the spell, and there is no arcane spell failure chance for wearing armor (since the ring user need not gesture).
For a randomly generated ring, treat it as a scroll to determine what spells are stored in it. If you roll a spell that would put the ring over the ten-level limit, ignore that roll; the ring has no more spells in it. (Not every newly discovered ring need be fully charged.)
A spellcaster can cast any spells into the ring, so long as the total spell levels do not add up to more than ten. A wizard could cast two fireball spells and a stoneskin spell into the ring (3 + 3 + 4 10). She could then give the ring to a druid, who casts the stoneskin spell from the ring and then puts four calm animal spells into the ring. The druid could give the ring to a barbarian, who could use all the spells but could not replace any.
The ring magically imparts to the wearer the names of all spells currently stored within it.