We already have eternal wands, and magical items that can cast spells a certain number of times per rest, depending on the number of charges they have. I suggest the implimentation of instrument items that are equipped in the weapons slot (most likely as a two-handed weapon) and used in the same manner as staves and other reusable "clickie" items.
Most of these items would have between 1 and 4 spells that can be cast, and a number of charges expended would vary depending on which spell is used.
These instruments could provide a bonus (of whatever type) to the perform skill, and could be found either as random chest loot (possibly in selected chests?) or as quest rewards. The enhancements that affect items that cast spells would affect them, and they would have a very high UMD check unless you happen to be a bard.
Depending on which spells could be found on these items, these instruments could give bards an alternative to UMD at lower levels, and save them some cash at higher ones.
As an alternative suggestion, have instruments be the bard version of Rune Arms. Wouldn't it be awesome to have a version of Greater Shout that with the same AoE range as a Bard Song?