This is very tricky. If your story works as is, simply expanding scenes will probably ed up in, well, expanded scenes. That'll drag down your pacing.
Most of my novels have grew by about 20k from their first draft. It was never my intention to lengthen the books, it just happened naturally when I edited. The kind of edits I made:
* In one book, a character who's a werewolf comes to the MC's rescue around the midway point. That's his first appearance in werewolf form. Since his character was rather underdeveloped, and it seemed convenient that there are only upsides to having a werewolf buddy hanging around, I added a scene earlier in the novel where he loses control and chases my character around the house. This added action, gave me opportunities to characterize both those characters, and I found a way to tie it into the overall plot.
* In a different novel, I needed to make my MC more proactive about achieving a certain goal. In the first version, she ends up at a location by coincidence, and while there, something bad happens by coincidence. In the second version, I had her contact someone (proactive!), set up a meeting at that location (no more coincidence!), and the bad thing that happens now happens as a direct result of that meeting (no more coincidence!). In addition, through her meeting with that person, I could weave in worldbuilding details that were missing from the early version.
* There was a lot of tension between my MC and her mother, but it never came to a head. In the most recent version, I added a fight between the two of them.
* My MC's father being in the hospital was a big plot point, but I realized later that that's all it is--a plot point. I made it more emotional and more personal by adding a hospital visit, showing their connection, strengthening the MC's motivations, and sprinkling in some world-building. It was a short scene, maybe two pages, but I think it helped.
So what I would recommend is taking a look at your novel and looking for opportunities to deepen your plot and characters. Is one character underdeveloped? Can you flesh out the relationship between your MC and their best friend? Does a plot event later on seem a little coincidental, or out of the blue? Does your MC have enough of a reaction to an important development? Is a plot point acutely dropped, or not established enough ahead of time?
There's nothing wrong with adding words, just as long as those words are CONTENT, and not PADDING. If they don't enrich your novel, you're better off not doing it.
In addition, 67k may be on the short side for YA, but it shouldn't be an auto-reject. Lisa McMann's WAKE is somewhere between 30k and 45k. Basically, I wouldn't worry about getting it up to 80k if you can't. Getting it up to 70k would probably be good if you could manage it, but again, only do that if it makes your book better. Don't add for the sake of adding.
Hope that helps!