Birthday on a Budget ~ Party Theme

What will the theme of your party be? With 5 children, boys and girls, we have had just about every type of theme.We have planned ahead and ordered things from Oriental Trading (you can buy summer themes on sale in the winter, if you plan ahead). I have gotten everything I need from the Dollar Store for a few parties.
    For the girls, we have had:

•  princess parties (dress-up items at the dollar store) 
• mystery parties (invisible ink pens and mystery solving sets at the dollar store ~ made clues to follow that lead to the goodie bags) 
• tea parties (tea sets at yard sales and good will)
• animal parties (stuffed animals at dollar store) 
• horse parties (also found at dollar stores).

    For the boys, we have had:

• cars parties (make cars out of cardboard boxes and have them wear them to race) 
• minute to win it parties (follow the blueprints and get  few items from the dollar store ~ ping pong balls, dice, popsicle sticks, coins, candy treats for junk in the trunk game) 
• paintball parties (for my teenager, we set up a paintball course in the woods and made sure everyone was covered and protected and had an umpire monitor the game) 
• buzz lightyear parties (made recycled jet pack out of 2 liters and cardboard wings) 
• treasure hunt party (made clues in Homemade Rocks around a park that lead to the goodie bags)
• Easter egg hunt party (I had a son born on Easter morning, so he has had many Easter egg hunt birthdays, but he loves the hunt) 
• nerf war party(I asked kids to bring their own guns, but we had plenty if they didn't have one. We bought extra bullets because most families know the bullets go where the missing socks live.

I hope you are finding this helpful. Let me know some frugal birthday theme ideas you have.