History Fair topic ideas can be difficult to develop and you shouldn't rush into it. Here are some sites to help you out.
Start here, at Chicago Metro History Fair. It's a lot of text but has some very valuable information. There's also a list of topics on their site here: http://www.chicagohistoryfair.org/history-fair/history-fair-a-nhd-theme/nhd-suggested-topics.html
Next, try the NHD site. Scroll down to "Choosing a topic".
Ms. Hawes also has some good info up at her blog.
And the Senn HS site has a lot of great PowerPoints, links and resources.
And the Senn HS site has a lot of great PowerPoints, links and resources.