Guitar and Music Theme Party

When my guitar-player hubby (now ex) had a birthday and we had very little cash, guitar picks were always welcome. They're inexpensive and always needed, either through loss or breakage. Many guitar-centred items, including picks, strings, sheet music (blank and printed,) soft and hard cases, and even guitar-shaped key rings are available at most music stores, whether brick-and-mortar or online.
Just a thought on party food; several kinds of tortilla chips are guitar-pick-shaped.

Don't know if you shop online, but there are some really cool mylar inflatable balloons out there. There are some of the regular round ones, but also 3-4 foot long guitar shaped ones. Ebay has them or some of the party supply websites do as well. Just do a search on "guitar balloons" or "inflatable guitar". The really neat thing about them is that they stay inflated for a long time (at least 2 yrs). You can also deflate them (if need be) then blow up again with a straw inserted in the small opening. Good luck & have fun!