Other than loving on the RO's? (Which all of them would do, of course.)
Emma: Become the best DAMN DETECTIVE THE WORLD HAS EVER SEEN!!
She’d also spend a lot of time looking into cold cases, and learn everything she could about anything and everything supernatural. She’s got the time, after all.
She’d probably stick with the agency for the foreseeable future.
Cassandra: Read. Everything. Indulge herself in her hobbies, and acquire many more in the process. Maybe open an animal sanctuary at some point. She’s pretty happy to live her life slowly, so I think she’d honestly be pretty down for the whole eternal life thing.
Hayden: Do some balls to the wall batshit crazy daredevil shit. When she inevitably got bored, she’d probably just travel. I see her coping the worst with it, due to a mixture of boredom and the lack of stakes. (lol) She enjoys getting up to dangerous things that get her blood pumping, and since she’d pretty much be immortal…it’d take away the thrill.
On a more serious note, she’s always wanted to have a bunch of kids, and she would not put herself through that if, say, they weren’t immortal as well. Also, watching the people around her get old and die would weigh her down more so than the other two. (Though Emma’d be a close second on that one.)