Another one I did before I started this site.
No meat for a month — sort of. I/we ate fish (my wife took part in this one). See, I am kind of a “meat and potatoes” guy so I’ve got to have some pretty straightforward protein sources and fish is barely an animal so I let it slide.
Funny. What kicked this one off is that we had recently visited some friends for a weekend and they grilled a lot of sausages, steaks, chicken (meat!). It was all delicious but we were stuffed solid and tired of solid food. It was such a natural time to do this 30 day experiment.
Big Lesson: I felt “lighter” on the no red meat diet. I can see why people go vegetarian because “I just feel better.” I will still make fun of vegetarians who cite animal cruelty issues, though. 
Big Effect: A great awareness of some good vegetarian dishes at restaurants. When making meals at home, we changed the way we plan a meal. We used to start with the meat portion and then build around that. Even now, we are much better at just putting some food together and occasionally leave meat out of it completely.
Difficulty: Easy to Medium. Easy in that all you have to do is not eat meat. Any difficulty came from having to be “those people” who complicated the food selection process when hanging out with friends. “What does everyone want on the pizza?” ”Well, we are not eating meat so can we impose ourselves on all of you?”