Machi Koro + Harbor
… works well as a two player filler. Not deep, but some thought involved.
Variant I now will always recommend — There must be at least 4 blue/green buildings numbered 1-6 in the opening. (For more players, I may make this 2n). It’s brutal to go last and not have any purchase that improves your position, ditto for second round.
I also removed 2 sets of purple buildings (for 2 player, I should probably remove one more). I’m tempted to remove a few Cheese Factories (and maybe Flower Store) since these only score as multipliers based on a single building (in the current set). So often they are worthless. I may just ban those from the opening setup.