A very simple method to add an unique look and feel to an area without spending a lot of development effort on asset creation is to **use a different color palette for each area of the game**.

In a 2d game, you can achieve that effect by doing a palette swap of your graphic assets. Just slightly change the colors in your preferred image editor. Or if you want to keep the asset size small, see if your game engine can do that for you at runtime. 

In a 3d game, this is easily achieved by using different light colors or a different color-grading post-processing effect for each area.

Don't be afraid of being brave in your color selection. Caves don't need to be brown and metal corridors don't need to be grey. The classic Super Metroid on the SNES is actually a great example of this. If you compare screenshots from different areas of the game, then you can see that many use the same tiles but with different color palettes. They were very bold in their color selection. The effect of the unusual colors makes the caves of the alien world of Ceres look, well, alien. But you also see a couple tiles which are unique to specific areas. This can also be used to give individual areas an individual aesthetic.

In addition to changing the visual aesthetic of each area, you can also try to aim for different **aesthetics of play** for each area. Try to have each area focus on a particular challenge of your game. Some examples of gameplay themes you could give to an area are:

* Precise jumps
* Well-timed jumps
* Well-timed running
* Lots of weak enemies
* Few but powerful enemies
* Enemies encountered at close range
* Enemies encountered at long range
* Enemies which surprise the player by appearing when the player wouldn't expect it
* Moving hazards
* Traps
* Predominantly vertical traversal
* Predominantly horizontal traversal
* Lots of secret passages
* Lots of puzzles, preferably of the same general theme
* Focus on one specific special ability of the player (very useful if the player just obtained that ability)
* ...and more...

I am looking forward to playing through all the different areas of your game.