... and making the players keep count. Just an outline but enough of one some folks wouldn't need more.
A big part of RPGs is looting and scooting and a major limiting factor on that is encumbrance or more specifically how stuff folks are carrying tires them out and slows them down.
The oldest school solutions were to assign a speed relevant to armor worn and to reduce that speed even further based on load carried (counted out to the coin). Big problem here is all the fiddly accounting and lack if payoff outside movement speeds which folks only ever really pay a lot if attention to inside resolving combat.
In recent years a popular alternative is slot-based encumbrance and a host of associated penalties with or without the movement penalty for armor worn. The slot system is pretty ideal as it it just requires an entry and there yuo go accounting all done for the object, but of course some folks can't stop fiddling away and before yuo know it there's big items that take up more than one slot... don't do that.
Use slot based load tallying. Armor is 1 entry (unless you use multiple pieces of armor in your campaign and this itself is a useful way to incentives players in seeking ideal and custom equipment) and works fine like that. I've worn armor for days and it gets hot and while its heavy it doesn't actually slow you down at all (at first). Make helmets and sheilds separate items from the rest of the armor even if yuo don't use pieces to calculate armor class, they are more of a pain in the butt and more significant than many game systems account for. A container of small items is a tallied item (nobody should worry about the load difference of 20 sling stones vs 24 sling stones) if a container is so light itself it can be folded up and shoved in another container it collapses down to a 0 load item. So with 12 items you have a load of 12.
Making that load matter.
1. If the load tally exceeds a characters Strength score the character is encumbered. They can still run but they can't sprint and their move is 3/4 regular. Over a distance a person who is encumbered is winded, being winded hampers all physical activity. (-4 to physcial actions including attack rolls in my campaign, disadvantaged in other rules). When a character is winded their movement rate is halved (that's half of 3/4ths).
How and when to check if a character is winded... make a strength check vs the load carried. Tally up that load an that's the number a player has to meet or beat on a d20 roll to avoid being winded. Check after 5 rounds of running or combat or any climb that take more than 1 round. You don't normally have to check if the character isn't encumbered.
2. Load total is the target number to start to sneak or quickly hide even when not actually encumbered yet. The more stuff you are lugging the harder it is to be stealthy. I recommend a d20 roll with relevant modifiers in old school campaigns. If encumbered penalties apply to this roll. Everyone wants to sneak and hide now and again and having the load tally also be the number to meet or beat everyone's going to be paying careful attention the load carried. This will have more impact on play than virtually any other form of encumbrance I've ever seen and enforces real world concerns of keeping to a light load while sneaking about.
3. A character can carry any amount over their encumbrance as they wish. But if the total is twice what would encumber them they are now burdened. A burdened character can't run and suffers all the penalties for being encumbered and they now have a based speed 1/2 normal. A burdened character has to check and see of they are winded any time they increase their load or following each round of combat.
Burdened characters can't climb or swim either. They also can't sneak.
Burdened character can hurt themselves. Any check for physical action that botches (a roll of 1 on a d20 when rolling high) harms the character (I recommend 1 to 3 points of dmage), this is how people kill beast of burden.
Special conditions for extra heavy things should apply: carrying another whole person is automatically encumbering, tryign to drag away a barrel full of wine all by yourself is a burden.
So in conclusion: Keep encumbrance and load tracking easy. Have that load carried and related encumbrance matter at multiple points in play.