Kids like instruments and making noise with them. And, perhaps ironically, they get really quiet and attentive when you start playing an instrument.
I play guitar in a lot of my storytimes. Always starting out with what it is, how many strings it has, what it is made of… etc. After the storytime I let kids play the guitar a bit, holding down the chords as they strum.
Even having one song you play every storytime as either a goodbye song, or just a song you always play, works well. Children at your storytimes will get to learn the songs you repeat, and always look forward to you playing the guitar at storytime.