Yes you can do Disneyland in one day, BUT you need to be there as close to park opening and stay to park closing. For the first few years I only had 1 day in the park as part of a longer Californian vacation. Last year was the first time I had 2 days, and I will have 2 days agin this year.
The plan for our one day trips was
arrive around 9 am and head straight for the main park,Star Tours, Peter Pan, Haunted Mansion, Pirates Indiana Jones
Lunch at Plaza at 1 pm and over to California Adventures, do The Little Mermaid, Soaring Radiator Springs Racer, and Toy Story Mania
Return to Main park between 6 pm and 7 pm, do 1 ride either Peter Pan or Pirates, get something to eat and watch the fireworks at 9.25
After the fireworks the park empties hugely but in June stays open until midnight. We used this time to do a second round of Pirates, Star Tours and anything we didn't do in the morning.
We leave souvenir shopping until the last hour in the park, as The Emporium on Main Street stays open until 1 am in June
It was hectic and we were always on the move, we didn't do character meets, didn't see the parade unless we happened to be using Main Street to get to somewhere, other than lunch, ate on the go, and used Fastpasses wherever possible.
We usually did about 15 hours in the park