You have the greatest input needed with costume ideas and that is, your ideas. You do not really have to buy or rent a costume if you do not want to. You can use your own innovative ideas and make a completely unique costume. The first step is to know about the situation. You would not want to have some glittering and jazzy costume for a formal dinner. On the other hand you do not want to dress professionally and formally for a hip party.
For a fancy dress party, you probably have the maximum liberty to run your imagination wild. For example, you can play Pinocchio in Mystery in the Library with a cardboard pencil nose and tying it on with white or skin coloured elastic. A heavily built person can colour his skin to become Aladdin's Genie. Think of the historical character or a character from your favourite Shakespearean play and start preparing the "period-specific" dresses and ensembles to match with it. Sure, this will be an exclusive costume, which no one can copy.
Be imaginative, innovative, and creative with your fancy dress costume. At times, it is just a perfect combination of your day-to-day clothing that adds value and fun to the occasion. A poster with a piece of hilarious message on your ordinary T-shirt is a perfect costume when you are going for a baseball match or for example, in Death on the Mound you can play a clutz by simply wearing different coloured socks.
Have you ever tried playing fun with your kid's watercolour tubes? Dress them up in a casual shirt and shorts. Draw a big moustache, put a hat, a holster, and see how deeply your kids love their new looks. It is not always the funny bone that has to be tickled with your costume ideas. In fact, costumes can make greater impact on your viewers if you plan to make them aware of certain issues.
Take for instance eating healthy food. Dressing up a child as a tomato or another vegetable and then explaining the benefits of eating fresh vegetables will get the message across in an entertaining way. This is often better than reading some boring old textbook.
Your costume ideas in a sense can add value to your life. It is a reflection of your attitude toward life. If you want to have fun, design a costume that will take you out from your busy life. If you are mad about Discos, the dance craze of the 1970s, you can reflect the same carefree mix of fashion and fun with your costume. As an earnest fan of Marilyn Monroe or Sherlock Homes, you can pay your tribute by dressing up like them. Plan your costumes for all occasions, be it Christmas or Halloween, in a different way. Don't buy expensive dresses and ready made costumes, instead, design them.