You also have to understand that a lot of people actually enjoy repetition, it's why techno is so popular.
It depends on the delivery as much as the actual action, to work off what was said above with the plant picking. There's a reward for picking them with improved skills then the ability to do other things with said skills but also it can simply lure a person in. The flowers is a good example of not necessarily a change of scenery (which is good as well as dylan stated) but the actual scenery itself. If it's a lush landscape that draws the person's eye, well picking flowers doesn't seem all that bad when you get to see how lovely everything looks while doing it, really. It's also an easy way to incorporate various different things since the players attention will be focused to a specific area, even if it simply is the ground!
Say for example you are, as you've mentioned a few times, watering crops.. The player's focus would be on the crops themselves and the ground, to work on the simple quest idea dylan said, there's a locket caught on one of the shrubs/whatever after some girl and her friends were strolling through the fields playing and laughing merrily. Voila a simple and very rpg friendly find and return quest. It'd be quite easy to add a bunch of "quests" in that sense, especially if you let your imagination go with a zanier ideas. Crop circles, molepeople, government conspiracies, you can even be a little controversial and base quests on topics like gm food. To be honest it wouldn't be too hard at all to create a whole bunch of quests for it. From simple things as say, going to the store to buy some milk to stopping crop circles and the subsequent alien attack to follow by genetically modifying your corn to have the properties of superglue.
Though then again that is highly dependent on the limitations of whatever it is you're using as well and probably going into far too much detail for a gmod idea, if I read your posts correctly.
Another thing which would be useful to do is look at current/old simulation games and how they try to fix the inevitable tedium that will follow.
Take the Theme Park series for example, particularly Theme Park World:
It's been awhile since I've played any of those games but the goals you had were simply to attain a new plateau of awesome theme park business man sir type person and unlock awesome new places to build your new unlocked fucking awesome eight mile long rollercoaster of awesomeness. The aims were all fairly simple and stuff like making all your visitors happy, having all their needs taken care of, earning over $100,000 in profit, etc.
You give one big global goal in this case; making like the world's like best theme park like ever! ..like.
Throw in a bunch of small things that work that idea and then you just build off of them, rewarding the player as he attains the small things and works his way to reaching that near unreachable ends.
Simple goals as in say: earn x amount of money over the next z amount of years, produce x amount of y crop in z amount of years, whatever and then just work everything around those very simple goals.
Regarding the throwing a wrench into the works idea, it is needed if it is going to be a simulation of any kind.. And I can't see any other reason why to have a game/mod/mode based on mundane actions since that goes against pretty much the whole idea of video games in the first place.
It will obviously break up repetitiveness though may as well frustrate the player if not done correctly.
In the expansion of Tropico they added random events to create a more difficult scenario (though the random events were completely controllable in how often they occured ranging from "why the fuck did you set it so high you stupid bastard" to "nothing") and your little muchacho would warn you when a problem was going to happen if it was forseeable... At least I think he did... Had a hurricane come pretty much decimated my entire little tropical paradise, millions of dollars worth of buildings gone and the USA is like, "oh hai there, here's ten grand to help get you back on your feet
" Yeah thanks, jerks. Maybe I can buy a used car and run over the remnants of my now sick and homeless inhabitants.
Any ways!
If you add something like weather for example, you could always make it so the player has the ability to know about it beforehand (newspapers, tv, whatever) and have a way of protecting themselves although not completely at least partially from the effects... You know, tv states that there's war is coming to our/your/his/their shores so he builds a bomb shelter and stores all the crops he cares for and has named, like gerald the potato.. just throwing it out there.. you know, in there to protect them. Though war isn't exactly weather.. I don't know, tornado, hurricane, volcano, virgin sacrifice, mohican rain dance. Anything.
I doubt this is useful or can assist you in any way as most of it is probably common sense any way, so sorry. It's late here; I'm tired, slightly batty and trying to be helpful. I don't think it worked. Proceed to laugh at how useless this epic length post is.