I was looking through the manual, but I did not see anything that looks like the ability to rest and quickly restore health and spell points. Is there any way to do this without idling the game?

There are quite a few ways.

Health:

1. Potions of Healing (heals 5d10 damage)
2. Symbol of Flesh (Gold ones heal fully + cure poison, red ones heal 3d10), only usable at Healing Altars
4. Vampiric Mist of Innate Weakness (requires 384 mana), life-leeching aura which only works on living enemies
3. Blessed Weave of Healing spell (requires 78 mana, received in the beginning)

*EDIT* 5. Hallowed Staff of Elder Wood (found at Elder Tree) improves Blessed Weave of Healing by 100% when worn

Mana:

1. Magic Plant (restores 10d10 spell points)
2. Jet Void Half (restores fully) & Ivory Void Half (restores 50d10), only usable at Magic Altars

Also the more Power your character has, the faster your spell points regenerate.

Why did they make red flesh symbols so weak? considering you even have to find an altar to use them.

For the same reason vampiric mist is weaker than four heals without a stick. AoD is not particularly well balanced.
Edit: not that it hurts the game much, it's just that "what the hell were they thinking" issues are annoying when I run into them, like typos. (note: I don't remember any prominent typos)

It is kind of annoying that the health and spell point restoring items use dice to determine the actual amount of healing/spell point renewal instead of constant values, when the whole healing system with nonexistent resting possibilities is bad in itself.

Limited health/mana potions, and limited symbols and altars. Since you heal over time, enemies don't wander far and I have frequent distractions, I'll often leave the game running while AFK and let health/mana regenerate while I'm away. That is limiting though since fif you want to continue playing, it isn't fun to sit AFK but something to keep in mind if you are stepping away for some minutes--don't pause the game but let it run.

Health and Mana management are a big part of the strategy of this game.

If you run low, you're not playing smart enough.

Usually you should gain enough mana while solving the puzzles or exploring the environment for the class you use, as it's based on your power stat.

The reason everything uses dies, is because it's based on the old versions of Dungeons & Dragons.

If you have a real problem with the dies, then savescum.