When I return something, I always just tell them I already played it, and I didn't like it, or that I already beat it, and I'll buy it when it's cheaper.
Doesn't really seem to phase them either way.
You could theoretically do that, but there is still a chance they could refuse you depending on your store.
I mean, many people are buying systems and consoles as gifts and what not, so theoretically there is a chance you could have it for that long and have not played it, so you could just return it, and say the recipient doesn't want it.
As long as you don't go busting through the doors in January, and be like, "Thanks for the free rental bitches." and do an interpretive dance in front of them while you ask them to give you a refund, you should be fine.
I would just double check with the policy when you buy it at the time, just to make sure you know all of the exceptions just in case, but for the regular 7 day pre-owned policy, I have never had an issue with returning an item at the end of the 7 days after playing through most of it before hand.