I agree with many here who have posted suggestions to visit your local library. Most have summer reading programs and I still remember this being a highlight of my summer when I was a child! Also, check with your local book stores like Barnes & Noble, Borders and Half Price Books--I know they all offer reading clubs in the summer time (some are geared only for kids in grades 1-5 though).
As an avid reader, I was sad when my daughter did not take to it right away, even saying that she hated reading! I struggled in trying to find the right balance to keep it a regular activity without being pushy. Last summer (after 1st grade) she really developed her love of reading. I let her pick a series and she read every single one last summer--like 35 books! They were in the "Young Cam Jansen" and "Henry & Mudge" series. What I found was that although she could read the "Magic Tree House" series on her own (which we also enjoy), she developed confidence by reading the other series that were a bit easier but she felt a much greater sense of accomplishment. That's what we needed--for her not to struggle through a book but to really read and enjoy the stories.
Definitely find a subject that she likes and find books in that area--your librarian will be able to help you. Let him/her know what kinds of books she is reading now and that will help them. My daughter is very tall (like me) and librarians at first would assume she was in 3rd or 4th grade when she was only in 1st and lead us to books that were too difficult.
One other thing to keep in mind, in my opinion, that just because a child is ABLE to read at a certain level does not mean she SHOULD. I remember kids in my daughter's first grade class last year reading novels that I felt just weren't appropriate subject content for a 6-7 year old. A friend of mine has two very voracious readers but she discovered that her first grade girl read a book last year that dealt with a girl's changing body, Santa, boys and girls dating, etc. My friend was not ready to have those conversations yet with her daughter! And I am probably the dissenter here, but I do not like the Junie B. Jones books. Perhaps it is the writer in me but the grammar is terrible (Junie's "voice") and I also do not think she treats the adults in the books with respect. Perhaps that is what is so appealing to the kids, but I could not get past that--maybe it's just me.... :-)
Oh! One more thing. There are a number of "regular" chapter books that have been made into mini-chapter books (in paperback) like the "Little House on the Prarie" series and even classics like "Black Beauty"-so check for those--they are a great way to introduce your child to some timeless stories without it being overwhelming. We also like any of the animal series books--Animal Ark, Pony Pals, etc. and also Cam Jansen, Arthur.
Good luck and have fun!