I believe AA and Delta both have their affiliated online shopping malls where you can earn miles for purchases at many national retailers. These malls are often the cheapest way to keep an account active. There's places in there you probably shop at anyway, so an easy way to get miles for stuff you already buy. In the case of AA for sure (not sure about DL), iTunes is one of the merchants, so as little as a purchase of a song or tv show can get you miles to keep their accounts active if there isn't something you buy regularly. For me, I usually do my semi-regular HP printer cartridge purchases through these kinds of malls to keep active whichever account needs it.

Note, in order to earn miles at any of the participating merchants, you need to go to the shopping mall home first, log in, and link to the merchant through that page (and have cookies in your browser active). This puts a cookie in, which is what links the purchase to your (or your kids) mileage account.