The word "adventure" comes from Middle English: from Old French aventure (noun), aventurer (verb), based on Latin adventurus ‘about to happen’, from advenire ‘arrive’. It concerns things happening. Drama. Conflict. As any great screenwriter, playwright and storyteller will tell you, there are a lot of ways to stage and set a drama - many different sources of conflict.
Let's look at some sources of drama. 90% of all drama and conflict is going to come from persons. The rest is environmental drama - floods, fires, wars, diseases, rioting, earthquakes, volcanoes, molasses tsunamis, and on and on. In other words, disasters.
So the drama and conflicts which come from a person can be powerful things to overcome. Let's look at a few core elements which drive bad guys.
Vanity: Arrogance; haughtiness; overconfidence; ambition; murder to prove a point; killing for oneupmanship; brinksmanship; and karening - calling in the law to harass innocents.
Greed: Avarice; miserliness; corruption; offering bribes; accepting bribes; loss of touch with reality; Marie Antoinette "Let them eat cake" (even though she never said it, the image is still used as a valid lesson); valuing things over people; social inequality.
Envy: Betrayal, after becoming a friend; murder; inferiority complex; poisoning the well; gossiping and smearing.
Hatred: Bigotry; self-denial; mass murder; nationalism.
Desire: An emotion almost never covered in roleplaying games. Lust; longing; stalking; obsession; crossing lines; ignoring boundaries.
Fear: The enemy fears the protagonists, and will do everything in their power to detroy them. If the antagonist is powerful, this cam be a problem for the characters - but remember that the enemy fears them? This means that the enemy is aware of their vulnerability - and fears that the characters can exploit that vulnerability, or flat-out destroy the antagonist ... if the protagonists can work out what that vulnerability is, in time.
Adventures begin when the player characters recognise the drama unfolding - the greedy tycoon sliding his grossly incompetent nephew into a position of authority with power over the player characters, or the group's "best friend" turning out to be someone who hates them after all, and has been feeding crucial intel to the bad guys all along - and do something about it.
Their plans can go awry - their plan of directly assaulting the stronghold of the bad guy who's been smearing their name is thwarted by a bunch of laws, and a whole lot of guards - and they may be forced to adopt new plans, reject them, and come up with even more plans; but it's the act of trying to figure things out, and trying to come up with solutions, and thinking up strategies other than combat, which make an adventure.
Moreover, the act of thinking on their feet, the uncertainty that they might fail and face worse than being reduced to zero hit points, is what makes adventures memorable. Opinion: I don't think you can ever find anything memorable about hack'n'slash dungeoneering without having a broader context for it. It's like eating mashed potato without salt or butter.
Edited by Alex Greene