With basic Post-Process Volume, you can make some simple effect to deco your game.
Bloom can create some glowing light on light color object. It can make the game looks more sci-fi with flare.
This might be the common effect of post process. It can adjust the color tone of your game.
Depth of Field
This effect might not be useful in 2D game thought it is really effective in 3D scene. Change the focus by adjusting the aperture and focus distance.
This effect can transform the corner of game view. The effect is common in magical background or mystery story.
This effect could make some dark corner effect which is quite common in retro photography or game project.
Post Process is the first key to decorate the game into higher quality. Although there are many effects would be more effective in 3D game, it is always a good start to get involved with visual effect.