The difficulty is handled in an interesting way in
Super Scribblenauts. The first time you solve a level, you merely have to find one way to solve it. Depending on your creativity and vocabulary, this may have a varying difficulty right off. However, if you return to the same level to attempt it again, you have to come up with not one, not two, but
three ways to solve the problem, none of which can be something you've already done on your first go-round.
As far as simply solving the different puzzles, some are more difficult than others, and sometimes it helps to think more specifically in order to be able to replay a level and still come up with answers. For example, in one puzzle, I used a "flower" the first time I played the puzzle. As I replayed it a few times, I had a hard time finding things I could enter that fit the same criteria, until I realized that I could enter specific types of flowers: tulip, rose, daisy... then I could "reuse" the idea, without reusing the word itself.
There will likely be a puzzle at some point that stumps you. When that happens, think about it for a while, perhaps create a flood or similar to kill everyone on the screen to let the frustration out a bit, then step away from that one for a while. When you return to it later, you might find that you've learned something in a different puzzle that helps you in the difficult one.