Simulating a journey
If I want to show that the journey was dangerous, but we didn't have time to play it out, I do this: For times I just want resources to be a little drained, I use this. (For example, the journey to a dungeon)
Remove half HP
Remove 1/3 of all resources like Spell slots, rage uses, and 1/day things. If in doubt, round favorably to the PCs. For 1/day things, let the PCs choose which ⅓ go, roll on a d6 (1-2 = lost), or say they are all unused. This also applies for magic items, feats, and other things with uses.
Total spell levels/3. So a level 5 wizard has 4×1, 3×2, and 2×3 spell slots, for a total spell level of 16. ⅓ of 16 ≈ 5, so remove 5 levels of spell slots in any combination (2+2+1, 3+2, 3+1+1, etc
Finally, take a short rest.
If you want resources to be moderately drained, like if you expect only a little bit of exploration/other encounters can happen, I would do this:
Remove ¾ or all HP
Remove ½ of all resources, as described above. If you roll, it's 1-3 on the d6.
Take a short rest.
For times that I think the PCs would be very drained, I use this. (For example, skipping the dungeon and going straight to the boss)
Remove all HP except 1
Remove ⅔ of your resources, as described above. If rolling, it's a 1-4 on the d6.
Take a short rest. (Heal!)
Immediacy Don't let the monk/warlock/whatever use any short rest resources to heal or regenerate before taking the short rest. That kind of thing is accounted for already, and removes some of the scarcity this intends to provide.
Why take a short rest? Fighters, Warlocks, Monks, etc can feel really bad since so much goes off on a short rest. Moreover, classes like Wizard and Druid get things back on short rests, and this is intended to similate a longer adventure day than we played. It makes sense that there would be a short rest in the adventuring day.
Why let players choose what they remove? Many players would be strategic with their spell slots and resources if they played the adventure out - saving powerful spells or abilities when they can. Letting players choose helps keep this aspect feeling like it's still their choice of what was used.
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