If you would like your game of Plumb or Spelunk to feature a skill system for its characters, wherein they may distinguish themselves from one another by more than their ability to run, jump, and attack, then you are in luck: Spelunk, and thus Plumb, already feature skills such as Spot, Use, and Resist, which fulfill many of the functions that a larger game's skill system would provide.
However, to flesh these games' potential out to their fullest, you may include three additional skills to the original set:
BEING SMART, which covers:
Knowing or remembering things
Being scholarly
BEING SOCIAL, which covers:
Telling people what they want (or DON'T want) to hear, so you can get what you want
Getting people to tell you what you want to know
BEING SNEAKY, which covers:
Hiding from dangers
Lying to people
Much like the other skills, these are purchased at character creation - all players start with a 1 in each of these three new skills, and can increase their score in them by putting in more points, or by finding treasure. Characters can also become limited in these skills, by taking the following modifiers:
BEING SMART
This skill handles any kind of knowledge or intelligence that your character might use to overcome challenges.
No Ancient (-2) Has no knowledge of ancient things.
No Advanced (-2) Has no knowledge of modern things.
No Monsters (-2) Has no knowledge of monsters.
No Traps (-2) Has no knowledge of traps.
Imperfect (-1) Doesn't function on 10% of all knowledge.
Unreliable (-2) Doesn't function on 25% of all knowledge.
Slow (-2) Takes three actions to use.
Good Memory (+1) You automatically remember any detail you knew was important at the time.
Perfect Memory (+3) You automatically remember any important detail, even if you didn't know it was important at the time.
BEING SOCIAL
This skill handles any kind of inter-personal interactions that your character might use to overcome challenges.
Monster-tongue (+3) Can try to be social with monsters.
No Bad News (-2) Cannot tell people what they do not want to hear.
No Information (-2) Cannot use skill to get information out of people.
Imperfect (-1) Doesn't work on 10% of all people.
Unreliable (-2) Doesn't work on 25% of all people.
BEING SNEAKY
This skill handles being devious or using subterfuge to overcome challenges.
(Note that being stealthy generally requires you to not move! But if a monster's Spot is not greater than your Being Sneaky, it can't see you.)
No Lying (-2) Cannot use skill to lie to people.
No Hiding (-2) Cannot use skill to hide from danger.
Slow (-2) Takes three turns to be hidden. (You are hidden starting at the end of the third turn)
From the Shadows (+2) Can use a basic Strike and stay hidden.
Soft Step (+2) Can move as if Slowed and stay hidden.
Silent Running (+5) Can move at full speed and stay hidden.
Ninja Leap (+1) Can jump and stay hidden.
Pickpocket (+2) Using an action, if your Being Sneaky exceeds your target's Spot, you can take an item from the target.