If you really want to blow the masquerade...

Lasombra: take ballet lessons...

Nosferatu or Tzimisce: go to a fancy dress ball and wait until midnight when everybody is expected to take off the mask...

Tremere: take your shiny new gargoyle out for "walkies"...

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Brujah: Become pissed when you drop your car keys, and lift the car to retrieve them...

Gangrel: Well... If you went bestial, simply walk around. If not, howl at the moon, lick your nads in public, pet police dogs in front of their owners, stand there taking 9 mm shower like if it was made of peanuts... You get the idea. Avoid furry conventions at all costs.You may not stand out enough in such a company.   

Samedi: Do whatever in public. Apart from pretending to be a roadkill, that is.

Salubri: Stare. Like, a lot.

Toreador: Go to museum and prove you are an author of an artefact. Yeah, the 500 years old one.

Ventrue: Go to EU Commission and propose something sensible. Then convince everyone present to follow through with it. Worldwide publicity guaranteed.

Tzimizce: Leave your house for once.

Giovanni - Go to an evening funeral and make the "guest of honor" talk to his family.

Ravnos: Steal something famous, like the Mona Lisa... and get caught.

Daughters of Cacophany EDIT: Give a concert to deaf people. (Melpominee enhanced of course)

Followers of Set: During a drug raid, walk out to the morning sun. Preferably going west.

Lasombra: For 4 or 5th Generation; blot the sun at noon, plunging entire city into darkness. Then call prof. Dawkins to the scene to explain it away.

Edit: Sorry, 3rd and 4th Generation only.

Duncan.Oliver wrote:

Ravnos:  Steal something famous, like the Mona Lisa... and get caught.

Or just do the classic "Cthulhu Fhtagn"; I'll let you work out the details.

Feral wrote:

...Ventrue: Go to EU Commission and propose something sensible. Then convince everyone present to follow through with it. Worldwide publicity guaranteed.

Impossible. Even with Dominate and Presence 10. They are professional politicians, so nobody can persuade them to something sensible. And even if it were possible to dementate them into accepting something sensible, you never get the British to accept something the French and Germans agree on...

Ventrue: Get overrun by a truck and when the police arrive file in the lawsuit yourself.

Giovanni: Open a restaurant in the family crypt and employ the residents as waiters.

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Tzimisce: Hold a housewarming party. In the middle of a bustling city. Where the visitors become the warm house part.

Brujah: Participate in a marathon. Cheat with Celerity.

Lasombra: Hold a press conference.

Nosferatu: Hold a press conference.

edit:
Salubri: Have your eyes checked. All of them.

Brujah: Catch the motor cyclist who has stolen the old lady's handbag, nicely bend his bike around the next signpost, and make the lady kneel in awe when you return her bag.

Lasombra: Drive a car and get shot by a red light or speed camera.
(Lasombra seems to be a very convenient clan. You can blow the masqerade by publicly doing nothing nearly as easy as any Nosferatu, Samedi, or Tzimisce. And that while you still look (and smell) mostly human.)

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Salubri: Work in a doctors or Psychologist clinic and have the highest recommendations for "miracle cures".

Follower of Set: Be a snake oil salesman (pardon the pun).

Malkavian: Have your limbs cut off via Railroad tracks, and when everyone goes to see what's wrong, leap onto the platform while screaming "T'IS ONLY BUT A SCRATCH!" while healing your limbs.

Any clan: Walk around during the day

Any clan: Visit a doctor without trying to blend in

The Masquerade is quite easy to break, by any vampire who chooses to do so. The question is always: How hard is it to repair the breach? The older Cainites in the Sabbat know that they don't want the Masquerade broken, any more than the Camarilla. Yet they also know that a good way to make the Camarilla waste resources is to send a pack into a Camarilla city, and have them sow chaos. The threat to the overall Masquerade is minor, as far as the Sabbat is concerned. All the Sabbat has to do is send a covert team to watch the Camarilla repair the damage, and they can uncover which humans and agencies are under Cam control. Then the Sabbat can target those people/agencies when they attack the city.

Gangrel: shapeshift on TV live show.

Tzimisce: go Zulo on the city square. Or turn to mobile pool of vitae.

Giovanni/Samedi/Nagaraja: organize Zombie Rights Watch. And make Zombies go on a riot in front of City Hall.

Lasombra: use Obtenebration publicly. Like, in a solemn celebration of something other than Halloween.

Tremere: start to levitate in a car park. If anyone asks, use Path of Levinbolt.

Tzimisce (Koldun only): Do Wall of Fire on a dance floor.

Ventrue: give pep talk on importance of abiding the law while being shot by muggers. Then make them surrender with Preasence.

Daughter of Cacophony: sing high C in a greenhouse.

Follower of Seth:
-Hand your heart over at the reception desk for safekeeping in the hotel safe.
-Use Skin of the Adder, go to the doctor and ask for treatment of your skin condition.
-Seduce someone at a club and when it's time for kisses, use Tongue of the Asp to make it more interresting.
-for a bit of fun without necessarily breaking the masquerade: go to the zoo, wait until you're the only visitor in the reptile house, use Form of the Cobra, and exit...

Any clan or ghoul: arrange for the public demise of an elder or really old ghoul

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Childe of Malkav wrote:

Follower of Seth:
-for a bit of fun without necessarily breaking the masquerade: go to the zoo, wait until you're the only visitor in the reptile house, use Form of the Cobra, and exit...

...And talk to some kid named Harry Potter on the way out, so he's convinced he can speak with snakes.