If you are not a programmer, or don't have one in your team, it might present a problem for the development of your mod. The next course of action would be to use other mods as a base (like Featureful or Opposing Force). However, in some cases, you may not be able to use custom code either. This is where this guide comes in: it contains a list of cool Half-Life features and stuff that you can implement in your mod!
Most of these went unused, or are workarounds rather than features themselves. It also contains some cool map tricks (such as the screen tint). These are meant to solve or tackle specific problems.

Caveats

The techniques discussed in this page isn't without gotchas. Here's some of the main problems of no-coding workarounds and dummied out features:

• They tend to be VERY specific. This means that they aim to replace certain problems, like the lack of a special entity that isn't on vanilla Half-Life, a variation of certain models, etc. You are not going to be able to add new weapons, NPCs, or similar to your mod, unfortunately.
• They are also very complicated and time consuming. Implementing them in your mod will be like trying to build a fully functional motorbike out of empty soup cans. Yes, it's possible, but not quite simple, and it can escalate pretty quickly.

UI/Menu

Main menu music

A common feature in a lot of mods. Add an .mp3 file named gamestartup.mp3 to YourMod/media. This file should contain your selected main menu music. It will be played every time the game is launched. However, the music plays in a loop and does not play again when the player leaves the game or disconnects from a server.

Mapping/Level editing

Screen tint

This one was created by the user Mota. The original map presents a México-yellowish tint, but you can change this to other colors to fit your needs. Explanation and steps for achieving this effect is available in the vault entry page, linked below:

Moving sprites

You would have seen sprites that moves through a level in various locations in Half-Life: Opposing Force. It is achieved through a custom entity (func_spritetrain) that are not available in vanilla Half-Life. However, you can use a trick to have a func_train display a sprite. This trick requires that you use some version of ZHLT compile tools (or its derivatives e.g. VHLT) that supports the use of the zhlt_usemodel keyvalue.

The following vault item has an example and the steps of achieving this trick: You can also use this trick to have a simple rotating sprite that doesn't use animation frames (thus saving on filesize), by using the angular velocity (avelocity) keyvalue. To have the sprite rotate in place, make the path_corner entities very close together (e.g. 1u apart) and have the func_train move very slowly e.g. 0.1u speed.

Model-based func_ entities

Using the same zhlt_usemodel keyvalue as mentioned in previous section, you can force a func_ entity, which is usually brush-based (just like func_train in previous section) to use a model. This is actually used in Condition Zero: Deleted Scenes with the mounted guns: they are func_tank entities that uses a model, that are otherwise identical in function to the vanilla entity.

The tutorial page "Using models for health/HEV chargers in vanilla Half-Life" explores the use case for this workaround with the health and HEV chargers in particular. A vault item is linked in the page, but here's the same thing:

Advanced Half-Life FGD

This FGD implements plenty of dummied out features from Half-Life, such as Dynamic Lights, the inclusion of new unused entities like item_sodacan or grenade. Keep in mind, this FGD is meant for Hammer. Other programs, like J.A.C.K. editor, can't really handle some features, and cause bugs.

Models

Different models for a single entity

You can make variations of pickups, monsters, etc. by adding a bodygroup to the model and adding the "body" property in a map editor: You can also add "limitless" body/skingroups to entities that already use them like HECU grunts or scientists. I wrote "limitless" because there is still a limit on every model. According to The303.org, the limits are:

• 32 submodels
• 32 skingroups

A problem with this workaround is that entities that already work with bodygroups and skingroups (like the scientists and security guards) can't have more groups added, since this will break the models.
An example can be found here, by UrbaNebula.

Misc

(LEGACY) Different launch screen

Half-Life uses a single background image both for the menu and when the game is loading. However, you can add different backgrounds for the loading screen and menu. This can be achieved by modifying the file YourMod/resource/BackgroundLoadingLayout.txt, and adding your new background files. A recommended way to organize the backgrounds is putting the files in different folders, rather than putting everything in the background folder: