For solo mandolin, I'd learn some tunes with a lot of double stops. I like to play LITTLE DARLING PAL OF MINE. BILE THEM CABBAGE is a cliche to mando players, but it's a good solo tune. People also like tunes that they can identify. A few years back ASHOKAN FAREWELL was a good tune. I have a medley of 3 Breton tunes that work well, but I don't have titles for them. Good luck and have fun busking.