Sandals like these: http://www.travelgearblog.com/files/2009/04/chaco-sandal-300x300.jpg
Are my big no-no. They make you seem older, and as safe as it is, not the best way to go if you want to look casual.
Flip Flops like these: http://www.anglerscenter.com/Merchandise/Clothing/Guy%20Harvey/HSL20110GH.jpg
Or just regular Old Navy flats are fine, because they give a great summer feeling. They make you seem fresh and young.
Last tip. BIGGEST NO-NO?
WEARING SOCKS WITH SANDALS.
I don't care how old you are, do not, please, ever, wear socks with sandals. Just wear shoes if your feet are that cold.
Good luck!