I just looked at the link for IXL math.  I had never heard of it.  Might be something I consider for over the summer.  My kids already get a math book they have to do over summer and return to school when they go back, so this will be a great addition.  Does it allow a child to move along above level too?
As another source, my DD loved the Flashkids Gifted workbooks.  They have a lot of logic problems and analytical thinking vs. just basic math problems.  The reading and english were good too.  I got the books at B@rnes and N0ble.  We got them two years in a row.  I found the basic summer bridge books much too easy, but these, since they were intended for gifted, were a little harder (not hard, but fun and colorful, and somewhat higher than the grade level books).