Graduations are times to celebrate hard work and bright futures. You should first select a graduation theme. You could tailor the theme on something the graduate enjoys. For example, if he or see enjoys football, you could tie the graduation in with football, with a football cake, decorations and everyone's favorite jersey.
If the party is for a high school graduate and the young man or woman is attending college. You could make it a college themed party with college banners, etc.
Some high school graduates would enjoy a graduation party at a fun, theme park such as Disney World, Kings Island, Six Flags, etc. If you want to limit the cost, have a cook out at a local park with a swimming pool, pedal boats, etc.
Unless the party is a surprise, involve the graduate so you make sure you are planning something he or she will enjoy.