cropped pants, sneakers, a cute t or cami and a zip up hoodie (think juicy or something) depending on whether or not you're going to be by the water or if it will be cold.
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"i tell you one lesson I learned
If you want to be something in life, You ain't gonna get it unless, You give a little bit of sacrifice, Oohh, sometimes before you smile you got to cry.." -The Roots
If it's just a casual "for fun" bike ride, I'd wear some casual crops, and maybe a fitted cotton tee and some cute sneakers (NOT flip-flops, i always bust it when i bike in flips).
Maybe something along these lines
With a colorful scarf in your hair?
Have fun!
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Know first, who you are; and then adorn yourself accordingly.
-Epictetus