"Account activity" for this purpose generally means anything that affects your mileage balance, up or down, by any amount. Things like address changes are generally not "activity." While each airline writes its own rules and can define activity any way it wants to, it would be too easy for people to extend the lifetime of their miles indefinitely if all they had to do was create a minor variation of their address every 35 months or so.

Several suggestions for ways to earn miles have been made in previous posts. Check your airline's Web site or the link posted by ANDREWCX for its list of partners. Some of the suggestions made above work for many airlines but not all of them, and the OP didn't say which airline(s) s/he is interested in.