Not all of the airlines below require activty to keep your miles alive. See this post for mileage expiration policies.
Often times redeeming miles is safer than getting miles for doing an action, as you don’t always get miles in time to keep your miles active.
You can actually get some pretty sweet deals, like The Economist for 3,200 miles for a subscription that usually costs about $100. Delta miles don’t require activity, but still 3,200 Delta miles for The Economist isn’t bad at all!
Or you can just get a cheap magazine for 300 miles to keep your account active.