Get one of the Marriott branded credit cards. This will get you some point activity. I do believe the base Signature card comes with the 1st year free. So you won't even have any cash outlay to get some point activity (and you'll get some nice bonus points plus a free night cert.) Another option is to buy something from the Marriott mall.

With that said, Marriott hasn't been very on-top of expiring points from inactive accounts. So they may not actually expire the points should the inactive time period elapse. But with that said, Marriott could at any time begin enforcing their expiration rules, so while it is nice that the points may not actually expire, it would be a major pain if Marriott actually did so.