I did the US savings bonds, but my funds were limited back then, so it only amounted to some thousands of miles before they stopped it

There was a small bank that offered Visa gift cards for purchase with credit cards for no fee, but I found out about it late and only made about 30,000 miles or so. At that time I could take them directly to my CU and deposit them, which made it pretty easy -- buy online, get them in the mail, take them to the CU and deposit.

I did the coins, but only in a limited way. My CU didn't like it when I brought a lot of dollar coins in, so I took to putting them in the vending machines in the basement at work and then pressing refund and getting four quarters back. I think I made around 50,000 miles on that.

When Microsoft was pushing the Bing search engine, there were word games you could play online and gain points, which you could redeem for airline miles. I made over 100,000 miles on that, but I was cautious -- I only did it a little for quite a while, then later ramped it up with multiple logins, bots to play some of the games automatically and multiple computers ( I had access to a computer training room after hours). They later caught on to people doing this and restricted it, but if I had been bold from the beginning I could have gotten lifetime Platinum easily on AA.

The biggest thing for me was the debit card/money order/deposit cycle that is still ongoing. I could earn over 100,000 miles/points in a month, although it was a lot of work. I still do it a little when I'm back in the US and have a CC bonus to go for, but the amount I did in the three years before I retired got me several years of business/first class tickets back to the US and stays in some nice hotels.