Simple ways to improve your map visually
Here are some simple ways you can use to give your map some more personality.
Remember to keep things in moderation, too much of anything is never a good idea. Try finding the sweet spot.
If anything is unclear or if you have any questions, just ask! <3
Screen effect / audio feedback
The idea is to give the player a feeling of accomplishment for i.e completing a certain stage or reaching a checkpoint. Turn the volume up so you can hear the heartbeat with the visual effect on the screen (recording issue).
The way I did it was by creating a "point_clientcommand" entity which is an entity that can send inputs to the players console, making only the player that triggers it be able to hear it.
Here's what the trigger looks like.

Notice the "AddContext" having a 0.3 sec delay. This is to prevent the player to be able to activate the trigger again by using a "filter_activator_context" as the trigger/filter is set to disallow players with the context "Lvl2" from activating it.


The easiest way to find a sound effect is to create an ambient_generic entity, find the sound you want the player to hear and copy the path as shown in the image below, then simply add an output like in the image above.

Particles and ambient sounds
I have added a running water texture on this ladder in the image below with an ambient_generic (sound entity) with a water dripping sound that loops placed below the ladder in the water.
I've also placed a particle with a water splash effect to give it a realistic effect as the dripping water hits the water below.


Point_spotlight
Adding a point_spotlight entity under a lamp does wonders visually. These can and should often be paired together with a light_spot entity to get more of a realistic spotlight effect.


3D Skybox
A 3D skybox can be used in a bunch of different ways. Take Vertigo for example, it uses prop_dynamics as moving traffic as well as env_sprites on the police cars.
This is a very simple example of a 3D skybox that still adds a lot to the map. I made a box shaped room as seen in the image below, adding a sky_camera entity inside it along with only two particle effects to create the moving clouds and sporadic lightning flash effects.


You also need to line the box up with your map to avoid getting only a portion of the sky with the 3D effect.
Just tinker a little bit with this and you'll quickly understand how it works.
Note that this is as basic as it can get, there's a whole world of stuff you can do with a 3D skybox.
Custom loading screen
A custom loading screen is just simply a good idea. It gives the impression that you really care about your work, which players will feel too. I highly recommend doing this as it is such an easy thing to do.

Link to Redmoons guide below on how it's done.
https://docs.google.com/document/d/1gj9dmCfYYPz4LJKHcgvQamyrTle1DoyVGapKS5YlgvU/edit
Color correction + env_tonemap_controller

The way to create a filter for color correction is to add -tools and -insecure to csgo's launch options and then load up your map, you should then see a new little menu where you can adjust all kinds of things related to color and brightness.
The saved file will end up in csgo/materials/correction as a .raw file.
env_tonemap_controller can control the amount of bloom (glare from lights) and the overall brightness/tone of the map.

The values being used here are specifically made for my map and would most likely be very dark if used without a lot of lighting from other sources, you should experiment with these values to get a tone that suits your map.

Without color correction + env_tonemap_controller:

With color correction and adjusted settings for env_tonemap_controller:

// xoxo, sinnes