As far as monologue topics usually science based stuff (biology/chemistry orientated), autism related stuff, health/physiology related stuff (human, animal or plant), or anything that is of interest to me at the time. At the moment it might be the local council and their ineptitude. Actually, there are a scary number of things that might trigger a lecture. I realise that people are often not interested but I sometimes don't care because it seems so important that they know anyway lol!
Interests lead to collections. In chronological order:
Horses - too expensive to collect a lot of live ones so I used to just chuck out the other animals in my farm sets. Eventually I moved on to expensive china ones and a few real ones.
Candles - thankfully I was still young and my parents were able to limit the numbers although I still find it hard to walk away from a nice candle.
Chooks - things with pictures of chooks, real chooks (I have 4), this has endured my whole life.
Cats - I found my feline soul mate at age 20 and discovered the delights of these creatures.
Christmas lights and decorations (I have a whole secret storage shed full - never used)
Plants - the type varies with each renewal of this interest.
On the whole I am ashamed of the magnitude of stuff I have collected and try to limit aquiring more because once I have brought it home getting rid of it is soul destroying. I am slowly culling my collections though.