Admittedly I have a nostalgic connection to Covert Action as I played it fairly extensively as a child however it holds up even after 20+ years. The procedurally generated cases offer a good amounts of variety in gameplay options and it has action and puzzle elements and a good range in difficulty scaling between the four difficulty settings. The action sequences have the right balance between stealth and action with a 2D overhead view that feels rather arcadish (especially if you get discovered). There are electronics puzzles that are similar to "complete the pipe" type puzzles where you try to get electricity to certain points by swapping tiles (and in harder difficulty you have tiles that can't be swapped and energy generating tiles that will generate current if they have no current going to them to increase the difficulty). The cryptographic puzzles are at the simplest level simple replacement but increase in complexity at higher difficulty levels. The only real weak point is the frustrating "driving" mode -- it is almost impossible even at the simplest difficulty level and winds up being downright frustrating at higher difficulty levels where you can't just rely on using the other game modes to get information for your case. None the less this remains one of my favorite games and even now I break it out at least once a year to run through a few cases.
