One extra mission that stands out from the rest is the Spider Lair.
For more immersion, it's best to google where each extra mission fits in the story and play it during your run. There are a lot of extra missions accessed separately from the menu: 9 unique missions played from the perspective of different characters and factions you meet in the main story. Spec Ops Reds are hunting you in several locations and you go full stealth w NVG on them. The pacing and the variety of the levels/missions are great. It's like the developers saw what fell short in Metro 2033 (broken stealth, cumbersome gun play, weird story telling, lack of background music) then fixed it in Metro Last Light Redux.
