Naruto is exuberant, brash, inattentive, and heedless to formality or social standings. Naruto has a number of childish traits, such as being a very picky eater (he almost exclusively eats ramen and a frequent customer at the Ramen Ichiraku), and being afraid of ghosts. However, whenever someone dear to him is in trouble or the situation calls for it, Naruto can be very serious, and will instantly try to come to their aid.
He also seems to have an on-and-off "friendly" rivalry with Kiba Inuzuka.
There have been about seven scenes in the anime involving him gaining weight, each lasting for variable periods of time.
About midway through Episode 53 of the series (Part 1) Naruto gorges himself on a huge watermelon he brought for Jiraiya, and gets bloated.
Fairly early on in episode 56 (Part 1), Jiraiya takes Naruto out for ramen (though he ends up bailing on him). Six bowls later, Naruto has a full belly, which he is happily massaging.
In episode 86 (Part 1), Naruto buys a ton of food at a festival. This is followed by a scene change showing him sitting on a grassy field, an empty bag near by and his hands on his stomach, which is bulging noticeably.
In episode 36 of the anime (Part 2/Shippuden) Naruto eats his fill at a hot spring, after which he is shown rubbing his bulging stomach.
In the Shippuden episode 95 omake, Naruto participates in a ramen eating contest alongside Sai, Kiba, Shino, Yamato, and Hinata. Although Hinata ended up winning, Naruto downed 30 bowls, after which a rather noticeable bulge was visible under his jacket.
In Naruto Shippuden 183 (which happened to be a flashback episode to Part 1), Naruto sneezed on Sakura, causing her to get mad and slug him. Depressed, Naruto gorged himself at Ichiraku Ramen and ended up with a massive belly. It should be noted that, out of the known weight gain instances with him, this is the rarer ones where his bare stomach is visible and his belly button is inside out.
In Naruto Shippuden 309 (which is also a flashback to Part 1), Naruto enters a Bean Jam Bun eating contest along with Choji. However, Naruto became too full durring the first round and subsequently lost.