Education.com is a great place to find science fair ideas.

Here are some projects related to earthquakes:
http://www.education.com/reference/article/Ref_Earthquake_Related/

Here are some projects related to the the environment and energy:
http://www.education.com/reference/article/Ref_NEED_Science_Fair/

And here are some related to the solar system:
http://www.education.com/reference/article/solar-system-science-fair-project-ideas/

If none of these are interesting to you, or simple enough for your student(s), check out this section of the site that has many more ideas...
http://www.education.com/topic/great-science-fair-project-ideas/

Good luck!

A fun science project is to make a homemade tornado http://www.easy-kids-science-experiments.com/science-experiment-ideas.html which shows a funnel contracting and lengthening, just like the real thing.