Seems like a lot of time has passed since you asked this and you did finally get one general answer. Your question is vague however, which makes it hard to answer. The topics of agriculture, transportation, and communication are HUGE. You need to narrow your question and then maybe someone will answer in a more timely fashion. Good luck!
Have you perused the science fair project ideas on Education.com? Link below. There are numerous project ideas there that would meet your requirements, and you can sort by grade and topic. I'm also including a link to some guidelines for successful projects. Good luck, and thanks for asking!