Mark Of The Ninja is a pretty great 2D stealth-em-up, but they recently did an "HD remaster" and tripled the price while removing the perfectly acceptable OG version from sale, but its still relatively cheap at 35% off.
Brothers - A Tale Of Two Sons is exactly as long as it needs to be, at like 2 hours long, and is 80% off and like $3? Its not exactly 'challenging', but its pretty satisfying mechanically.
Also like peeps above are saying, there's a fucking ton of last gen titles that are still perfectly playable at like, 75% discounts on their base $20 pricetag - the F.E.A.R series is pretty fun