The Greenbelt Mini Maker Faire is a smaller version of the national Maker Faires held in New York and San Francisco. It's not exactly Steampunk, but there may be some crafting and electronics projects that are inspirational for costuming or props. I'm hoping that this doesn't turn out to be a totally kid-centric affair. We went to the New York Maker Faire in 2011 and it was great, but the 2012 one was more focused on student projects. So far there are a few craft type things listed but the full schedule wasn't posted as of Thursday, March 20.
Here's a link to the website for more information.