Recently played Red Dead Redemption, and the game has a thing that would be awesome in Adventure Mode.
It's small missions where you need to gather a number, like 7-10, of a specific plant. It's surprisingly addictive to roam around the countryside looking for the plants.
Dwarf Fortress has a huge number of plants that could be used for more than decoration.
I suggest to create quests where you need to locate a type of plant, grab it and return it to start: "Go find me 6 leaves of Mountain avens and ye shall be rewarded".
I had huge amounts of fun roaming around the DF-countryside, looking at nature, and running into trouble and weird stuff on the way. BUT it would be even more fun if you actually had an in-game reason for the character to travel to some remote savanna or tundra and look around.
I think it will be so much fun that I suggest just adding it now with a placeholder system. Add naturalist scholars that should be fairly common and place them somewhere easily accessible like in human and dwarven taverns. They're just standing there pestering people to get them plants for their studies. You talk to them, they give you the quest to get some randomly picked plants. When you bring them back he says he's happy for them and he asks you for some more.
Further down the road, plant-gathering missions could be connectied to whatever functions plant are given - if they have a use for magic, the mission-giving NPC could be an assistant to a wizard needing plants for some spell. (Actually would love if necromancers needed plants for their horrible experiments.) Or if medicine is introduced, it would be doctors asking for plants to try them out in treatments. It could also be connected to religious rituals, like the Grand Orange of a religion needs random plants for the religious stuff.
As for the reward for the quest - well the usual reward of mission in adventure mode is a pat on the back, and I'd be completely content with that. Perhaps a new reputaton title like "Explorer of nature" could be added? This would be more than enough motivation for many players I think.
Bonus 1: The same thing could be done with hunting animals. "Get me 6 hides of giant cheeth."
Bonus 2: As a high-tier difficult version of it, there could be asked for underwater or underground plants. These should not be mixed in with the normal surface missions. Maybe when you max out your rep of the surface missions or did 20 of them, the NPCs would then switch to underground plants and underwater plants. Locating a cave entrance and managing to get in and find your way back out with 5 specific plants would be SUPER challenging to me - much more than killing a megabeast.