You could write about a huge event or person that impacted history. Make sure that you can gather plenty of information. Also, dont forget to make it intersting and knowledgable, it is best to pick a topic that not many know much about so you can amaze them with the information.
THE MOST IMPORTANT THING TO REMEMBER IS TO PICK A TOPIC YOU WILL ENJOY DOING...DO NOT PICK A TOPIC IN WHICH YOU WILL BE BORED DOING.
You could find ideas at historyfairprojectiideas.com