Always like to use magic mouths for warnings, riddles an shit like that. Good spell that playesr later gets access to.
Locked massive doors being pounded on, sealed by powerful runes and forbiddings.
A grave, tomb, or sarcophagus with a hero lying in state. +1 sword, an intreresting curse, or just backstory to round it out.
Shadows acting weird, twisting and screaming in silence or escaping you to form unknown letters and words.
Physics free zones, water dripping to the ceiling from the floor, a black pool above your heads.
The bones, equipment (good starting loot) and maybe journals of a previous adventurer.
A clue written in Futhark or something that players can't decipher, later give them access to another piece of writing that lets them learn runes and riddle out the message. Like translating the map at beginning of the Hobbit, make em feel smart.
Give em a treasure that they have no idea about, a small bone die with skulls for pips, a brooch shaped like a clenched fist, a headband that grants strange visions, backstory or abilities to discover later.
Sneak past massive numbers of superior foes, see an army passing from hiding or something similar.
A corridor where armed and armoured skeletons stands in recesses in the walls bare inches from the players who are squeezing past.
A hidden resting place or secret area where the players can rest, recuperate and learn all that crap thats most important for spellcasters. Think the guardroom in Moria where Pippin drops the stone.
A ghost or haunt wailing or acting out a sisyphean tasktotally oblivious to the players.