I'm not an Expert (maybe the @Unbounce-Experts can add some more value to this), but I can share what I know.
First off, I advise that you stay away from "ads". Domains like "ads.yourdomain.com" are blocked by extensions like AdBlock and I'd argue, don't inspire a ton of trust.
Be creative. Pick verbs for your subdomain that are related to the action you want your users to take. Some great examples are:
That's as much insight as I can share at the moment, but the Experts will likely have more to say on the subject.