Here's a simple way to make a big impact with inexpensive balloons. Blow up or buy helium balloons in a rainbow of colors or another color combo to match your party theme. Tie the balloons to string or thin ribbon at even intervals. Leave enough room on the ends to tie the string to a low anchoring point on both sides. Hang the balloon banner at the entrance of your party, in the backyard between two trees (as an alternative use metal stakes like the ones that come with a tent), or frame the cake table with the balloon arch.