Taking advantage of a library's summer reading program is nearly always a great idea. Not only do most libraries have goals kids can meet, but many also have one or two special programs a month where they talk about books and offer enrichment or entertainment. If a library has a science fiction theme, for example, they may have a program showing an episode of "Star Trek" or something similar. The summer reading programs also allow children to make friends with others and have something in common with them.
The old SRA kits schools used to have were also a great reading incentive, since it was a feather in one's cap to have read enough over the summer to start at a higher level in the kit when school started. Some libraries also offered them as reading challenges, and this was great, too.