The best thing to do is get on the MAX light rail or a bus (Portland is noted for its great public transportation), get off at Pioneer Square, aka Portland's living room, and look for Shelly's cart. It's usually near Mr. Portland, the statue of a guy with an umbrella. You may have to stand in line for a bit if it's lunchtime, but Shelly's quick and her burritos are worth the wait.
If you think a Honkin' Huge burrito is too much for one sitting and you didn't bring a friend to share it with, there are smaller sizes available. Also, she's willing to cut a Honker in half and package it up so you can have lunch today and tomorrow. Chat with her while she puts your burrito together - she's very friendly.
The burritos are vegetarian, but even if you're a meat-eater, they're so tasty you won't want to pass them up. You can get them with sour cream or yogurt, tomatoes inside or outside, and a choice of sauces, mild, spicy, or hot.
There are other lunch carts in downtown Portland, but I usually head for Shelly's Garden. Come to think of it, I'm hungry!
(First published on Qassia. It's what they call "intel" and since I really like Shelly's burritos, I thought I'd share. And I really am about to head downtown to get one!)