@bdw inspired this thread.
So I tried using O!ePay within the Octopus app for the first time yesterday with my wife's NFC-enabled Android phone. We could deduct at least 500HKD from each of our personalized Octopus cards that are linked to her PrimeCredit UnionPay EarnMore card, which earns 2% on all purchases in HK including AAVS, which as far as I know is the highest available (please tell me of a card that earns more).
SC's Simply Cash card earns 1.5%, but PrimeCredit's card is still better and worth it, in my opinion, even with the annoyance of no login website (we still haven't downloaded their OmyApp) and their website and online application forms only being in Chinese. I wait for their paper statement in the mail and use Planto (I like it, but it has issues) to check on the PrimeCredit spending.
With O!ePay, you can set up FPS with a bank account, and you transfer money in and out of your O!ePay account. If you transfer 1,000HKD (two personalized Octopus cards linked with AAVS) that's a 20HKD reward for moving money from your Octopus cards to O!ePay and then FPS out to a bank account (unfortunately ICBC (Asia) isn't one of them, but HSBC, HS, and most other major HK banks work with O!ePay via FPS). It's definitely not going to make you rich, and maybe not worth the hassle for most, but if it's amusing to you and you treat it like a game, then I can recommend trying it.
@bdw mentioned figuring out a way to use O!ePay to pay his mortgage, which I'll assume is with SC. The O!ePay has a specific designated link to SC Mobile, so maybe it's even easier to use like a SC Simply Cash card, use a personalized Octopus card linked via AAVS with your SC Simply Cash, add up to 500HKD (maybe you can figure out how to do more in a day) via your Octopus card to O!ePay, and then transfer your O!ePay balance to your SC bank account, which pays your mortgage (I'm assuming as I don't have an SC credit card or bank account). The Octopus app mentions that if you transfer out more than 3,000HKD of your O!ePay to a bank account, you'll get charged a 1% fee, but if you earn 2% via AAVS then you effectively can keep earning 1% until you hit your credit limit and/or you give up wasting your time on this method.
I didn't get in on the game with using HSBC's Payme app before HSBC closed the loophole, but I guess you can still transfer 2,000 out of your credit card, move it to a mortgage linked savings account, and earn a bit of interest before needing to actually pay off your credit card. Again it's probably more hassle than it's worth for most, but if you like trying to play games to skim off a bit from financial institutions, then it can be amusing for some.
Any other ideas for skimming a bit off of financial institutions, but that are probably more hassle than they're worth? Credit card sign-up bonuses in HK are paltry compared to sign-up bonuses for many U.S. credit cards, but I guess getting credit card sign-up bonuses even if they're relatively meager can be amusing enough for some. The other issue in HK with credit cards is that they aren't nearly as widely accepted compared to the U.S. and plenty of other places that don't actually like the security issues involving handling lots of physical cash. If you only spend at places that take almost any kind of credit card in HK, then you could probably save a lot more money by changing your spending habits than playing skimming games.