1. Nature. I think this is what comes to mind for a lot of people. The Garden of Eden. Paradise. Clean air. Forests. Sparkling streams. If you type in "utopia" in an image search, you'll always find photos of green grass, blue skies, the sun shining, beaches and big, fluffy clouds. Fantasies often revolve around magical forests. It can also symbolize a refuge from the negative complications of modern life. Here's a picture from WikiMedia Commons that might stir your imagination:
|Mountain deep forest by Paolo Neo on Wikimedia Commons|
2. A high-tech society where there is no disease, hunger or pain. I love the possibilities here. Invent new technology for your world. Imagine how your characters spend their lives. Do they live forever? Do they work? The fun will be in creating the details. What does the food look like? How did this society emerge and is everyone happy with it? Since they've never known the hardships we endure - always living pampered lives without suffering - does that make them act spoiled? Are they genetically engineered? The possibilities are endless. Here's a picture to inspire you:
|FUTURE CITY by Boombastik3 on deviantART Some rights reserved|
3. A new planet, an orbiting colony, a refuge under the sea, a community on the moon, a domed city that keeps out the dangers outside.... Maybe your utopia isn't even on Earth. Maybe your characters live in a space colony that orbits Earth - or a settlement on Mars.
|Lunar mining, an artist's concept / NASA / Pat Rawlings|
Well, there you go. I hope this post gets you inspired to create your utopian world. Here are a few other sites I stumbled across that might help you: Utopia Definition on Wikipedia, a post on UphillWriting.Org about utopias and flashbacks, and a discussion about utopias on NaNoWriMo.
Edit: Here's an article about pessimism vs optimism in science-fiction from Smithsonian Magazine. The comments below are also thought-provoking.