I've never come across much stuff, but have come across interesting people/situations...
*Almost every morning on my regular commute, there is a professional dog walker with 5+ dogs on retractable leashes...
*On another MUP 20 minutes after, there is an older man looking at the ground while he smokes pot from a pipe every morning... A little further up there is a homeless man napping against a post with a hoodie on that covers his face. I see him every day - sometimes even on weekends if I'm out riding then. Always with the hoodie on - I've never seen him awake and never seen his face... For awhile, there was a middle aged women on the bike path with 3 chihuahua dogs, all off-leash, that would run after me and jump 3-4 feet in the air next to my bike while barking... She never seemed bothered by this... Luckily, I haven't seen her in awhile.
Random stuff:
A few times, on my ride home through a suburban marsh land path - I have ridden through police "busts" of homeless people camping out in the marsh. One time, a homeless person who seemed high on drugs ranted loudly at me because he thought I was the police (my lights were on at night)... When he realized I was a cyclist, he told me very politely to have a safe ride home...
I once rode past a big group of homeless people having a party next to the bike path - about 30 people, a bonfire, singing, guitar-playing, food, etc...
On the golden gate bridge, riding home from a job in SF, I passed a guy on a bike that had a cape and goggles on... and he had a backpack with a dog in it and the dog had a cape and goggles on also
The best thing I saw was not while biking, but it comes close - I was hiking and came across a naked guy painted from head to toe in gold paint (including his hair, pubic hair, etc.) and he was clearly high on a drug like acid or mushrooms and smiling serenely. It was a long hike and I was behind him for awhile...