https://www.gog.com/en/game/age_of_wonders_planetfall https://www.gog.com/en/game/age_of_wonders In fact I could link here also other AoW games (aside from AoW 4, which I didn't play), but I would especially recommend Planetfall due to sci-fi setting and lots of interesting features, as well as first game in the series, as it's incredibly cheap and gives plenty of fun.
https://www.gog.com/en/game/missionforce_cyberstorm It's not the greatest discount, but so far there wasn't any higher, and this game totally deserves attention, mixing accessible gameplay with great amount of HERCs customization and various features (like re-directing shields power), meaty campaign and good hot-seat... Plenty of good stuff.
https://www.gog.com/game/call_of_juarez_bundle Oh, boy. That price for all games from the series (excluding the Cartel), and they all have incredible campaigns - and I'm saying that as someone, who normally is not into western stuff. Plus multiplayer in Coj: Bound in Blood is quite fun.
https://www.gog.com/en/game/disciples_2_gold First game is not bad, but second is has more "everything" (especially in terms of units evolution paths), all those campaigns and single scenarios will need a lot of time. But it's worth it :)