Though it lacks the balloons and has a basement, someone built a real-life copy of Carl and Ellie's house, complete with a painting over the fireplace. In Herriman, Utah.
Still, a group of people actually did manage to float a house using helium-filled balloons — appropriately enough, for a TV show called How Hard Can It Be?
Easter Egg: On the main menu of the UK Blu-ray (and possibly on other releases), pressing the Left arrow on your remote control reveals "The Egg", a short piece in which the film makers discuss the abandoned plans to make Kevin's eggs into the elixir of youth.
The original opening involved young Ellie and Carl constantly out to punch each other due to a childish rivalry until they fall in love in their teen years. Then, one of Ellie's last acts in her hospital bed is to throw Carl an affectionate punch in the chest. Needless to say, Pete Docter said he got funny looks from test viewers.
Initially Carl would make a balloon animal of Kevin to lure Muntz into its labyrinth home, then abandon him there. This piece of Alas, Poor Villain was done away with due to Pete Docter feeling that it made the ending more about Muntz than Carl and Ellie.
Other endings were considered, including one where Muntz would redeem himself after talking to Carl; one where he got his foot tangled in the balloons and was pulled up into the air; and one where he ran into the house to grab what he thought was Kevin (actually just another balloon) and was followed by his dogs, their combined weight causing the house to fall.
Kevin was originally introduced with Carl and Russell finding her in one of Muntz's traps and freeing her. Then Docter realized it wouldn't make much sense after she'd been dodging his traps for decades.