Do a search on the internet for holidays. You will be surprised at what you will find. Everything from religious holidays to festivals. You might find local celebrations such as a Founder's Day for your town or a crop that brought on a festival such as the Strawberry Festival. You can use the standard holidays with great success! (Christmas, Thanksgiving, St. Patricks Day, etc.) There are usually lots of decorations available.
Check out holiday lists in the links section of Resources.
Genres can be a great way to get started in displays. You can highlight a general genre or be just use a section of the genre, such as gardening mysteries. Any of these can be taken right off your shelves and given a prominent place or a little corner. Don't forget you can do a display for kids, different ages...
An easy display to put together is a read-alike. An example of this might be "If you like John Grisham, you might like..." You might include a photo of this author, and a list of authors who have stories like or might write like this author. For James Patterson, I included in the display some caution tape and play handcuffs. Places to find read-alike lists are found in the links section.
Getting community groups such as 4-H, a gardening club, or even the beekeeping association to promote their groups at the library can expand into a theme throughout the library. Maybe the storytime folks will be doing a session on gardening at the same time the gardening club is doing a program at your branch. You can carry this theme onto an end display having to do with gardening. There can be a rolling display in the children's area about how vegetables are grown, and a bulletin board promoting all the gardening activities at the library and in the community.