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Binding of Issac and Rebirth are very nice, and there are tons of free indie games that are pretty fun. I assume that you are gaminf primarily on the Xbox though, so maybe try Red Dead Redemtion. There's quite a few things to do, it's not that expensive as far as I know, and it's good if you like shooters. If difficulty isn't something you don't mind, then at a stretch you might like the Dark Souls series too.