Too many to mention, but here are a few:
Archaeology - the first thing I wanted to do was dig up dinosaurs. I recall the teacher going round the class when I was five or six, asking each child what they wanted to be when they grew up. The littany of nurse and hairdresser for the girls and fireman and traindriver for the boys was halted by my admission!!
Insects - did projects on them and drew them endlessly, labelling their various parts.
Astronomy - still with me today and an Aspie classic!
Reading - Enid Blyton was my first reading obsession (always accompanied by a plate of salad to eat while I read them), then moved onto Nancy Drew, then got into Stephen King.
Snooker - wanted to make a list of the top 100 players rankings using info from the telly (this was way before the internet). Dad said, you won't do it as only the top 32 usually get on the telly. He was right.
Collecting shells and erasers - amongst many other things (fans, badges, beermats) then subdividing the collections into catagories. Freaked when my sister and her friend used some of my beermats (unusually shaped ones with holes in) to draw around and got felt pen all over them.
The Brat Pack - we move into my teenage years and the boy-related ones begin. Emilio Estevez and watching "Stakeout" repeatedly, as it was the only film of his the local video shop had. I didn't care.
Johnny Hates Jazz - short-lived pop group from the late '80s. Would get up in the middle of the night to watch the progress of their single in the US charts on the show with Casey Casem (sp?).
The X-Files - great show and Mulder is hot.
Star Trek:TNG - I have a thing for guys in glasses and loved Geordie, with his ultimate pair of specs. The fact he couldn't make eye contact with his VISOR may have some significance here. Taped every episode and collected many copies of magazines.
The Bill, House MD, Roger Black (I smuggled my VCR recorder on holiday so I wouldn't miss him racing in the European Championships), interior design, counted cross stitch, rugby union, the list goes on and on...