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.

The main problem 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. Warning: Copypasted text up ahead!

The tint effect is achieved by having an env_fade (screen_tint) repeatedly triggered by a trigger_multiple (mexico) covering the whole area it's supposed to affect.
Both the fade's "Hold Fade" time and the trigger's delay before reset are set to 1 second, so the fade gets re-triggered every second while the player stays inside the trigger.
That would be enough for a continuous tint that fades away when the player leaves the area, but if you also want it to fade in smoothly, it'll need a more complex setup - especially if the player can move freely between areas like in this example.
For the smooth transition, I used:

• A second env_fade (screen_tint_start) with "Fade From" disabled
• A second trigger_multiple (mexico_border) at the threshold between areas with a 0.5 second delay before trigger
• Two pairs of trigger_changetarget, named change1 and change2

When the player crosses the border, this is what happens:

• The threshold's trigger_multiple, mexico_border, targets the pair of trigger_changetarget called change1 after a delay of 0.5s
• Meanwhile, the area's trigger_multiple, mexico, targets screen_tint_start, as that is its initial target
• The change1 pair activates, making it so mexico now targets screen_tint every second from now on, and mexico_border now targets the change2 pair
• If the player walks back through the border, mexico_border will now trigger the change2 pair, which reset both mexico and mexico_border to their initial targets so this whole sequence can be repeated

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: