Oh boy. Here's a (very) short list of (really) old games that are great. Just don't expect modern graphics, most of these are sprite-based, however, so they aren't as horrible as early 3D games:
1) UFO: Enemy Unknown (a.k.a X-COM: UFO Defense).
2) X-COM: Terror From The Deep.
3) Jagged Alliance 2 (make sure to grab the fan-made v1.13 patch which makes the game significantly better).
4) Star Control 2 (there is an open-source clone called The Ur-Quan Masters with improved graphics and the ability to run on modern systems, as the original dates back to 1992!)
5) The Baldur's Gate series: Baldur's Gate, Baldur's Gate: Tales of the Sword Coast, Baldur's Gate 2: Shadows of Amn, Baldur's Gate 2: Throne of Bhaal. These things are absolute masterpieces of storytelling, gameplay and sheer scale. There are also "Enhanced Edition" versions of these that were recently released. They cost more than the originals, but they remove the need for a bunch of mods to fix issues with the originals (especially relating to graphical issues on modern systems). You can still use the old games, however, but the new re-releases are more convenient. The re-releases also have Android and iOS versions for tablets, but I don't recommend those due to the clunky controls.
6) The Icewind Dale series.
7) Knights of the Old Republic (the original from 2003, not the MMO). The writer for this is the writer for the first two Mass Effect games and the game is amazing. It should run on pretty much anything today including most iGPUs.
8) Knights of the Old Republic II: The Sith Lords. It isn't as good as the first, but it still blows out most of the games straight out of the water.
