Themes are something that have totally helped me organize our school days!! I love having a main idea to work with and off of. The first year I homeschooled I even coordinated our theme with the letter of the week....talk about a lot of planning:) Below is a list of some of my favorite places to locate themes and activities to go with them!
http://totallytots.blogspot.com/
http://prekinders.com/theme/
http://www.perpetualpreschool.com/preschool_themes/topics.html
http://www.1plus1plus1equals1.com/PreschoolPacks.html
http://www.everythingpreschool.com/