You could sell promos, some basic stuff, but gets you money. Also, depending on your audience, you could try promoting affiliate offers. As far as I know you cannot put links into tiktoks bio, but shorten the url and/or get a domain and display it in your video. Content locking works great for young audiences, I used OGads for this back in the days, although that was on IG and not on TikTok. You should be careful with this though, as it could get your account banned. I don't know about TikToks policies regarding it, but you are better off having a close look at it, as it would be better to have an account that does not make a lot of money, than having no account at all.
Depending on your niche you should just be creative in my opinion. Think of something that will not hurt your account in any way, like promoting scams, but still makes bank. I hope this helps in some kind of way, as I have hardly any experience with TikTok, but I see my little sister using it pretty often, so I could think about a few things.