A short list of things (d8 maybe) — good, bad, dangerous, or interesting — that can happen on the way from point A to point B.
Point A: The half-sunken remains of a great stone wall at the edge of the Great Swamp, on the edges of the civilized world, the protruding towers of which are inhabited by weary elves who stand guard, lighting campfires close up against the crumbled stone parapets at night....
Point B: The slowly, slowly swirling mud sinkholes at the center of the Great Swamp, beneath which are the stony remains of the huge colosseums, amphitheaters and vomitoria of the bygone Age of Marble.
In rough order of proximity from A to B
1. A tower on the wall, miles high, collapses backwards into the Great Swamp. Plumes of silt and water burst into the air. The stones of the tower float to the surface, conspicuously stable, and lead like stepping stones to a mud hut. Inside are feral young women. They are illiterate but prodigal magicians. They did this. If taught to write - way easier said than done, - their words could be spirit-consuming limited wishes (save or die, cumulative -1d4 penalty each time).
2. Beneath the wall, behind a cave-in, lies the service tunnel. The service tunnel. You could walk the whole way to the greatest lost coliseum of all, at the heart of the sinkholes. It is dry, which is nice, apart from concerning drips by buttresses of petrified wood. You will need to travel single file if you use it. Alternate entrances appear at five-mile intervals: ladders leading up submerged stone towers, capped with manhole covers.
3. A particularly dry and hospitable spit of earth is home to the lean-to of Jim the Elf. He works on the wall. He got a bit lost. He has been here for several centuries, living (poorly) off the land. He seems really good-humored about it. He tells dad jokes. If brought back to his post, the party will perhaps learn that Jim is owed several centuries worth of back pay, and maybe a workelves' compensation claim. The present budget of the guard elves' guild probably can't take the hit. The party might be able to get a cut.
4. Fires burn green here. No reason. Anyone asked about it will have a theory as to why, but they are wrong. All of the theories sound plausible. Even testable.
5. In the wetter parts of the swamp, Grandmother Catfish takes an interest in your party and your boat, which she will silently follow for uncomfortably long periods of time. Grandmother Catfish is 50 feet long and dead. She is a catfish. She is also a lich, a necromancer of legendary skill and art. She is very eloquent and a gregarious hostess. She will help you breathe and sleep underwater, if you want to crash. Company is very rare and her children, also dead, do not visit. She will feed you real good (think a Game of Thrones-quality banquet, but consisting of Louisianan comfort food) and not even murder you, because she likes company more than she loathes all living things. She wants someone else dead.
6. A rotting bridge perches in the swamp like a pier. It comes from and leads to nowhere. The ghost of a gladiator from the Age of Marble waits, as though for a ride. He can see the road. There is no road anywhere. He insists he can see the road.
7. No dry or even semisolid land for days. Rafting or boating necessary. Shreds of rock rasp holes in party transportation. Leaks perpetual. Restful sleep impossible on account of need for constant bailing, repairs, despair. Consequences of sleep deprivation (1d6):
1. Unable to memorize spells in the morning, or memorized spells are different/mistaken.
2. Exhaustion. No HP recovery from resting.
3. Equipment sodden and ruined. Lose rations. Leather armour rots. Spellbook pages stick together. Scroll ink runs.
5. Hopeless. -2 to saves.
6. Hopelessly lost and unaware of it. Going in the wrong direction until some landmark sets you straight.
8. Aqueducts reach out of the swamp overhead. They are crumbling and precarious, slick with algae. Degenerate orcs cling to their undersides like tree sloths covered in war paint. The war paint is mostly for show, since the sloth-orcs are genuinely pretty slothy, but they have bad attitudes and a lot of stick-to-it-iveness. If the party stays near the aqueducts, the sloth-orcs will slowly, slowly advance towards them overnight, traversing the aqueducts upside-down with spears in their teeth and malice in their eyes. They all want their first kill, which would be the first in the history of sloth-orcdom. If they manage it, the party's survivors will see the coronation of the sloth orc king, He (or she) Who Kills First, who will usher in a new age of slow-moving ambition. They will march the next day for the wall, which they have heard about in legend. They may spare the rest of the party in exchange for their services as guides.