Speaking from experience here, since my mother is currently working as a third-shift manager at a McDonalds near my home, I can attest that they typically don't mind people just coming in and using the free wi-fi, and they don't force you to buy anything. Nobody at the McDonalds knows who I am, let alone my mother's relation to me, and they don't bat an eyelash at people just sitting at a table in the lobby and getting on their wifi for whatever reason, be it netflix watching or the vidya gaems, so long as you're no disturbance to the other patrons.
And it's not like McDonalds food is -entirely- terrible. A plain hamburger is a dollar, nothing but bun and patty. It's thirty cents more to add cheese for a cheeseburger. Two bacon-egg-and-cheese biscuits, thanks to McDonalds' recent decision to start selling breakfast all day, are $3.33. Hell, just buy a dollar sweet tea or an ice-cream cone off the dollar menu, or like Astinus noted, you can buy the Happy Meal toys, if you really need to justify a visit to McDonalds.