It really depends on the country too though.
I presume in US you get alot of offers and trials.
In here I used to get a lot of offers to enter a mobile number to get a a confirmation code and fill a survey after that (sweepstakes, etc). So either google some mobile numbers you can use, or buy those cheap/free start sim cards to get codes (each number can be used once if it’s same offer company). I think I got like 1000 creds like that.
Edit – Also +1 on the trials, unfortunately I do not get many of those offers, and some do not accept my cards at all.
Also depositing on a Casino may be worth it for you. I did it once, when I wanted to play for 10 eur anyway, so even if I lose the 10 eur, I still get it back in gold. Win-Win.