DoubleDown Slots is a good one that also pays out instantly. I'm currently working on the last reward level of this one from the last double credit weekend.
Download and open app = 10 20 credits
Complete Level 8 = 2366 4732 credits
Complete Level 10 = 4550 9100 credits
Complete Level 12 = 7716 15432 credits
Complete Level 15 = 9260 18520 credits
Total = 23902 47804 credits
It took me from May 26 until June 9 to complete Level 12 playing about 1/2 - 1 hour per day. 2 things I like about these type of offers:
1) I've had issues recently with completing offers and them not paying out, so this one lets you know right away after downloading and opening if your 20 credits show up. If they do you are good to go, if not, don't waste your time.
2) There is an Auto Spin button on all the slots, so once you have a decent amount of chips built up you can just set a moderate bet amount, click the auto spin, and go about your business for 1/2 hour or so and really don't even need to interact with the game anymore for however long. I know many will shake their heads at me saying their time is worth more than playing a game an hour a day for free credits, but with this auto spin you really only need to take the time to open the app, click through the opening "offers" to buy chips, set up your game on auto spin, then you are free to do something else for however long. I can set it up in the morning and let it run while getting ready for work, then set it up again in the evening and let it run while watching TV or playing WGT.
It is a bit of a grind going from Level 10 to 12 and then even more so from Level 12 to 15 for the final 18520 credits. It's kind of like how in the WGT world where the XP needed to level up goes up exponentially above level 90. I completed the download and Level 8 on the same day (05/26), Level 10 2 days later (05/28). Then it took 12 days to reach Level 12 (06/09). I've been working on getting to Level 15 since then and only 65% of the way through Level 13, but it's a 60 day offer so I still have 38 days to complete.
Of course keep in mind all these game offers are only available on Android platform. I've always found it odd that all of these offers aren't available on IOS. Whenever I watch daily videos for 2 credits on my iPhone they are 99.9% advertising games to download, but when you go to the TapJoy wall on iPhone there are no games available.