The three main ways of adding replay value to a game are through Novelty, Creativity and Evolution.

Novelty

Content cannot all be seen on the first run.

• Branching Paths: Multiple game endings, choice making in quests that affect gameplay. The replay value comes from playing the game again and trying different options.
• Multiple Character / Classes: Unlockable characters, different play styles, character builds, stat allocation, varied types of equipment and weapons (that affect gameplay) to chose from. RPG games like Skyrim have immense replay value as different character builds will completely change the game experience.
• Procedural Generation: Randomly generated maps, enemies, loot will lead to different challenges with the same base mechanics which in turn will forces players to adapt and stops them from feeling complacent. Each run will feel unique. This approach is the foundation to Roguelike games. Roguelike and Roguelite

Avoid:

• Staleness: Instead of branching paths game has a strict linearity. Instead of unlocking new characters with different abilities, the game has characters set in stone or the new characters are just a visual reskin from the previous characters. This wont make a game worse but kills replay value.
• False Choices in Branching Narrative: Different choices that lead the player to the same place. Cheap and waste of time, the illusion of having a choice will leave those who play it with a sour taste in their mouth and wondering why there was a choice in the first place.

Creativity

Supports user generated content.

• Games with Concrete Goals: Game provides many tools / ways to get to an end but its up to the player to explore and achieve that goal as they see fit. Examples: Metal Gear 5, Hitman
• Sandboxes: This category of games is particularly good at adding replayability to a game through creativity. Platforms for the players to build their own story and create their own goals.
• User created levels: Games with level editors or that support mods.

Avoid:

• Inhibition: Game wont provide enough tools to experiment and play with.
• Overpowering play styles: If one play style is overpowered players will often not bother with trying something different. Limiting resources available can fix this issue as it will stop the player from doing the same thing over and over again and force experimentation.
• Bad curation of player created levels, offline servers

Evolution

Enjoyment comes from learning the game and getting better over time.