At my first-ever Con last year, my friend and I wanted something manageable to make and comfortable to wander around in, so we decided on civilian-dress superheroes. It worked really well for us--we each had something iconic enough that it was clear who we were supposed to be, but we were comfortable and could make most of our costumes out of regular clothes. No one seemed to have trouble any recognizing our characters.
I went as Tony Stark the engineer in the middle of working on his suit (rather than a complicated full-on Iron Man). I bought a T-shirt with an LED "arc reactor" inside, like this one
to wear with jeans and sneakers, plus one home-made Iron Man gauntlet. The gauntlet was the most complicated part of the costume, and was something I cobbled together out of painted posterboard, hot-glue, and a gardening glove (I also added an LED-light "repulser," but that wasn't really necessary to get the point across
). My friend was Steve Rogers in khakis, a checkered button-down shirt, brown leather jacket, and boots (based on the ending scene in the Avengers), plus a Captain America shield that she purchased. You might be able to do something similar for one of your favorite superheroes.
I also think Kaylee from Firefly would be pretty fun, recognizable, and comfortable. Get some coveralls and add a teddy bear patch to the knee, wear them over a flowery/girly shirt, and put your hair up in two little buns on the top of your head. Maybe a cute paper parasol.