Adventure mode plays like a regular roguelike RPG. At the moment adventure mode is underdeveloped and not nearly as deep as fortress mode is, but its very fun in its own, sick yet amusing way.
Throwing is bugged at the moment, so you can basically pierce someone's brain with vomit or break their bones with sand, but it can provide quite some laughs, specially when you kill a giant by throwing a fly at him
It also allows you to do everything you wanted to do with goblins and elves. Tired of elves bringing you useless thread and cloth? Go to the nearby settlement and break the druid's legs to teach him a lesson! Gouge their eyes out! Decapitate them and use the severed head to beat children to death! Strip them of their clothes and throw them back at their faces! Pinch their lower bodies! Stick a spear in their guts and twist it untill they bleed to death! Kill everyone then drag them near some trees and set everything on fire as a sacrifice to Armok!
Sorry, got carried away
You can visit other forts, civilizations, towns, settlements, caves, etc. You can also visit the ruins of your old fort, if it got conquered, destroyed or abandoned and loot everything you want, including any artifacts, and artifact weapons and armor can be very powerful.
You can also wield ANYTHING right now, so why not beat people to death using that legendary trumpet your craftsdwarf made? Or maybe throw that legendary anvil at an elf just to see what happens?
It can be very confusing at first, since adventure mode lacks much of fortress mode's interface, but once you get the hang of it, you'll play it at least once in a while. It may not be as addicting as fortress mode at the moment, but its being developed, and it is quite fun in its current state.