How do i control my hair in humidity?

So i style my hair as usual and it looks fine, i have naturally straight hair but since i added layers i have to straighten it. But when i step outside for a little bit my hair becomes wavy,poofy,and frizzy. tomorrow im going to be outside all day with my friends and i don't want my hair to become a mess! help! what should i do? also are there any cute hair ideas i could try without having my hair down?