You're not "with her," but you're also pretty sure Donald Trump isn't going to make anything great (again). You've checked out the two visible third parties, but neither Gary Johnson nor Jill Stein do it for you. There may be some other underdogs on your ballot — say, the Reform Party's Rocky De La Fuente — but you're just not sold. Maybe you've got someone else in mind you'd like to vote for, and are wondering if there's a way to do that even if they aren't on the ballot. Whom can you write in on Election Day?
Technically, you can write in a vote for anyone — your mom, your dog, perhaps a Deez Nuts throwback. Two caveats: 1) You can write in anyone if your state allows write-in votes (nine of them don't — South Carolina, Louisiana, Mississippi, Arkansas, Oklahoma, South Dakota, New Mexico, Hawaii, and Nevada), and 2) in 34 states, in order for write-in votes to be counted, the person you're writing in must have filed paperwork to run. My Time to Vote lists around 100 write-in candidates for president (including one "Mickey Mouse") who have filed paperwork with the Federal Elections Commission. But not all these candidates are valid write-ins in every state; candidates need to meet state-specific filing requirements to be counted on their ballots as well.
So if you're writing in someone who isn't running and hasn't filed paperwork, your vote might only count in the states that don't have rules around write-in candidates: Vermont, Oregon, Alabama, Iowa, New Hampshire, New Jersey, and Rhode Island.
However, even though write-ins are allowed for anyone here, there's some grey area in these states. Officials from Iowa and Vermont told Snopes that, though write-in votes would be counted in these states, no electors would be assigned based on them unless the person being written in has declared his or her general election candidacy. That means if you're still feeling the bern and were planning to write in Senator Bernie Sanders on Election Day (I'm looking at you, Vermont), your vote either won't be counted or won't amount to anything elector-wise in most states. Far from declaring his candidacy or filing paperwork for himself, Sanders has been campaigning hard for Hillary Clinton.
So in all but nine states, you can write in whomever you'd like. But if you want your vote to actually be counted, your options are narrowed substantially; you'll need to pick someone who met your state's requirements, if it has any. And even if it doesn't, just know that even if a lot of people write in the same person in your state, it might not distribute electors based on the write-in votes.