I'd like to know some fun challenges to play with a single Sim inside a story (no BACC or Legacy or that kind of challenges). What challenges do you think I can do?

The homeless challenge is the only challenge I know, and it just so happens to be a single sim.

Depends on the sim and on your preference. I have completed and enjoyed:
Polyamory Project Challenge* (Rules a point 4.1 here: http://www.gamefaqs.com/pc/914811-the-sims-2/faqs/34940 )
Bachelor Challenge* (Rules here: http://modthesims.info/t/412146 -- can also be played as a Bachelorette Challenge)
Internet Lover Challenge (Rules here: http://modthesims.info/t/335007 )
Asylum Challenge* (Rules here: http://modthesims.info/t/335041 )

I am currently playing:
Crazy Cat Lover Challenge (My version, rules here: http://esmeiolanthe.livejournal.com/44746.html )
The Sun and The Moon Challenge* (Rules here: http://whisper-doll.livejournal.com...ml?thread=28621 )
Nickle & Dimed Challenge* (Rules here: http://www.modthesims.info/showthread.php?t=450850 )
Sideshow Challenge* (Rules retyped here: http://esmeiolanthe.livejournal.com/129122.html )

I want to play but have not yet started:
Messie Messlot Challenge (Rules here: http://modthesims.info/t/335040 -- I think that getting the lot messy enough to be used would also be a fun challenge)
Toddler Mania Challenge* (Several variations out there -- Google to find your favorite)
An in-the-works crafting challenge (No rules yet, but basically a Sim will need to make one of everything that can possibly be crafted prior to death)

I consistently and repeatedly lose:
Permaplat Challenge* (Rules here: http://esmeiolanthe.livejournal.com/54275.html )

*Requires more than one sim but can be completed in one sim's lifetime quite easily

EDIT: Fixed incorrect link to rules. Sorry!

Oh..I should read the Challenge forums a bit more!
This looks very cool. I'll have to go create some clothing first..lol

It's a neat challenge, I like it. I'm not one to do a challenge and keep score and follow all the rules, but it's great material as story inspiration and how to make your game a little more difficult.

Aside from the fortune aspiration (that I'll change the rule of..) This will be interesting. I do not like fortune sims.

Fortune aspiration is part of the challenge because they'll be extra hard to keep happy with no expensive stuff.

Have you had a look at the tiny challenges for boring moments thread? http://www.modthesims.info/showthread.php?t=341154