Now, let us see what you can do if the budget is small, the room is the only thing you can get and there is no other choices available. And the ceiling is low. Or you have the party in your house and the living room and dining room is all you have. What do you do in this case?
First of all, I would suggest to think about the theme, in the situation like that it seems to be one of the things that is flexible. One of popular themes will be much easier to work with, than something very original. The reason is simple - there a lot of readymade thing things to be used, inexpensive enough to stay on budget. And then the theme can be supported with balloons, ribbons and lights of the appropriate colors.
Let me give you an example: a small party of 30 people to celebrate 5th birthday. The theme is Superman. They rented a room in a club, the room was nice, but ceiling was very low - no more than 7' and uneven, which made it even smaller. The mother of the birthday boy asked if we can create the sculpture of the Superman and centerpieces also with the hero. Yes, we can, but in order to be clearly recognizable, he has to be at least 10' tall. Even if we would create something 7' high exactly, it would look squeezed. Never mind that creation of such balloon sculpture would cost more than they planned. The solution was very easy: since it is such a popular theme, we created 4' columns and put the mylar shape balloon - the Superman - on top and placed the columns in the corners - if the eye has a reason to see the most spread points of the room - the room looks larger.
Centerpieces in the room with such a low ceiling is a very tricky thing - make it a bit larger than reasonable - and the space will be cluttered and unwelcoming. So we just kept the colors and made them small. The to promote the theme even further we used ribbons in the theme colors, napkins and even forks! But, with a little more money spend we could have plastic plates with the picture of the superman on it, as well as on the napkins.
Is this true only for the kids party? Not really. Say for a stylish black and white party one can use a centerpiece that combines black and white balloons, white flowers and dry leaves, colored black and white (I will try to find good examples and add it to the post).
So, as long as we include the room and the theme into the planning and start early enough, we win!