Maquis and Under Falling Skies are my main solo games at the moment. Both are worker placement, but UFS is a dice worker placement game, where you roll a handful of dice and then decide where to spend them. Maquis has a “resistance helping to defeat the Nazi occupation” theme and Under Falling Skies is basically Space Invaders re-imagined as a board game. Both are well done and play in 20 or 30 minutes usually.