I occasionally go through bursts of solo gaming, either just to spend some time away from a screen or to try out new games. Some of my favorites to solo are:
Zombicide - This is a "pick your flavor" kind of game, but I enjoy Black Plague/Green Horde. It takes up a ton of table space, and many of the missions are designed with higher survivor counts, so you have to be okay with controlling up to 6 characters with their own abilities, inventory, and tracker boards. If I'm down for a long solo game with minis and randomness of dice, I'll pull this one out.
The Manhattan Project: Chain Reaction - While the standard rules have players aiming for a certain number of points to end the game, the solo option is more of a high score race against yourself. The objective is to build as many atomic bombs in only one play through of the deck. Several of the cards have different rules for solo mode, but these are printed on the cards themselves, so there's not a ton of referencing the rules over and over to make sure you're doing the right thing in solo. I've enjoyed tracking my score each play and seeing if I can beat it (or at least match a particularly high one). This is an inexpensive game and plays pretty quickly solo, so its easy to get in more than one game if you enjoy it.
Black Orchestra - This is one I picked up just a couple of months ago, but I've played it several times solo already. The ultimate goal is to carry out a plot to remove Hitler from power within Nazi Germany either before or during World War 2. This involves forming a plot, gathering the items or intelligence needed, and then meeting the requirements to carry out the plot. It's a tense game with a heavy theme, but there's quite a sense of accomplishment when you finally win.
Duel in the Dark - I'm not sure how available this one is, but it's a fun game of a British bomber heading to a target within German-occupied Europe. In solo, many of the advanced rules are stripped out, so it's pretty light.
I'm sure there are others I'm forgetting, but these are always fun!