Theme: Vampires & Werewolves
Everyone has to tell a scary tale they have made up on their own; the scariest one wins the prize. Or, you could play 'Name the Vampire'. Here, you describe a vampire character with the help of hints. The person who guesses first, wins.
Theme: The Carnival
'Hoop a Loop' is another fun game; it game involves looping a hoop around your waist. Whoever does it the longest, wins.
Theme: The Hollywood Red Carpet
Ask all the kids to dress up as their favorite celebrity. If the celebrity in question has a very common wardrobe in his/her personal life, the kids can wear a costume s/he wore in a movie.
Another game is 'From Start to End', where you tell everyone the start and end of the movie in a gist. Whoever answers correctly first, wins.
Now that you have everything ready, the next important part is implementation. How you implement the theme you have chosen, will determine how good a host you are. Plan in time and you won't really have a problem. Best of Luck!