I am at the gym pretty consistently and I always find myself wondering what to do with my hair. Most likely it is going to be up so it doesn't get all sweaty and stuck to my face. I'll cover some options of different lengths.
Short and Textured: Think styling wax, messy but refined
Bob: not exactly short enough to get away with the messy look but not long enough to pull everything back. I reccomend side braids or twists
Shoulder length or longer any density: Faux Bob or a top knot a looped ponytail, this is already a really popular option, it only takes a minute or two extra to make it look really chic.
Long, medium to thick density: Braids are a beautiful way to showcase your long lucious locks. Here's a new one, braid up the back with a top knot. Pretty. Or how about a ponytail fishtail ?.
Let's bring back headbands! seriously, they are pretty fun to wear, they work with any length and can be combined with most other styles plus they are very helpful at the gym.
Obviously you aren't looking for anything too fancy but you would like to look pulled together and maybe even be able to wear the style out for the rest of your day. It's easy to rinse off after the gym and leave your hair alone. You can blow dry on low after a rinse at the nape of your neck and around the sides of you face where we tend to sweat the most. Or you can spray a dry shampoo at your roots and nape of neck, tousle your hair and go. Nobody will ever know.