You might look in the archives of Scholastic Book Fairs - there was one a couple of years ago that had one catch phrase - "Lasso a good book" or something to that effect. They have some good thematic ideas that you can use to jump start.
Our school had a rodeo theme for our Fun Night last year which is a little different, but some of the ideas might work if you adjust them a little. The chairperson took digital pics of the kids for $5 each wearing a cowboy hat about a week or two before the event and we put them on wanted posters that they picked up at Fun Night. You could somehow incorporate reading minutes into these wanted posters I would think, like in the description put "Wanted for reading x number of minutes" or something like that. We also had a mechanical bull which was very safe and the kids loved it! It has a bouncy trampoline thing around the whole bull so when they fall off, no one gets hurt. We did have to have them sign releases for that, but that was the highlight of the evening. You can have a book swap and call it a book roundup or something, give the extra books to charity. Let me think about it some more.
My school is a Title I school, and there are many efforts in promoting family involvement in the child's education. In Sept. we are having a family reading event called a Reading Rodeo. Faculty members will wear cowboy hats. Any ideas along the rodeo theme to encourage reading and promote literacy in the home?