I found 'Accurate scanning of QR codes', which might be Chelle's mention. 51 silver. Worked for me.
Lucktastic: A friend pointed me towards this. There are usually 2 different offers (pick 1). Now I see 100 silver for load/run, and 850 silver for a slightly more complicated load/interact/run that requires downloading an additional app as part of the interaction.
The 850 silver offer was pretty straightforward, although I needed to refer back to a screenshot of the offer to make sure I got through the steps correctly.
Ebates: currently 7200 silver. I know there's been discussion about this in other threads, but it hasn't made it onto the 'official' list. You need to install the app & make a purchase from a participating vendor through the link the Ebates app provides. You do not give your credit card info to Ebates-- payment sources are only entered in the vendor site (e.g., Walmart, Home Depot, etc.), and you can use any saved account information you already have set up at the vendor site. The minimum purchase is only required for the additional $10 bonus that is offered from the Ebates app itself, and is not necessary for receiving the CellNuvo silver.
You can install the app on multiple CN accounts, but I wouldn't recommend trying for the Ebates $10 cashback bonus on multiple CN accounts unless you can use different account credentials for the multiple ebates accounts, as ebates (justifiably) frowns on giving the $10 bonus multiple times to the same user. This is NOT an issue for qualifying for the CN silver-- just make one purchase through the app and let the account stagnate.
Walmart (Kobo) Audiobooks. 6000 silver-- this, and the direct Kobo app seem to disappear pretty quickly, but I've seen them return regularly, too. Load app, sign up for free 30 day trial (requires CC, and don't forget to cancel before 30 days.) First audiobook is free with trial.
This one hung as 'pending' for me. I followed sovashadow's suggestions for support, and sent screenshots of the welcome email and receipt for the audiobook, and received credit the next day.
I don't know if it's possible to do both app offers. I don't listen to a lot of audiobooks, but the 40+ hour one I picked is mine to keep even after cancelling.
I've seen an e-book version of both Kobo and Walmart offers, too, for lower reward.