One reason why many people don't put a ton of effort into activity based contests... is because of the "I-never-win-anything-anyway" mentality. So why not set up a fairly attainable milestone prize, that participants can automatically win, once they reach a certain number of points. It would help to push many folks to do more than just opting in at the top of the funnel to get the initial free gift.
This concept could even be extended to have several "automatic" milestone prizes that participants can "play" for, over the duration of any given campaign.
for this you can just set up more than 1 reward... each reward requiring more pushes
Exactly - that's what the giveaway campaigns are for (so it's already possible) :)