Mag - what age? I don't have any experience with Kumon or similiar. I would avoid the worksheets. I think the kids get so much of that at school and it's fun to learn other ways. (I put in some prices but you should confirm these to be acurate.)
ideas for writing:
Online writing -
http://www.ctd.northwestern.edu/gll/ with Northwestern University, 9 weeks, check this but I think tuition is about $400, they have some scholarships available. This was a good experience for my DS9 last summer.
Summer Journal - Here's my plan for this summer. I'm going to make a journal for my family and kids may write, draw or add stickers. In the begginning I will make a list of things I would like to do this summer and ideas of things to include. Things to put in the journal might be feelings about your day, what you did, nature sitings, or a poem. I won't correct any spelling or grammar unless asked. It will just be for the joy of doing it. My DD said she will make her own too.
Books to encourage writing: I just leave 1 or 2 at a time laying around and see what happen
You can write a story by Lisa Bullard
Art for kids cartooning by art Roche, he also has one with comic strips
http://www.loreenleedy.com/ "Look at my book" is good.
Library Mouse by Daniel Kirk, This can inspire writing.
Math ideas: Search the library for fun math books on multiplication.
Math potatoes : more mind-stretching brain food / by Greg Tang ; There are many of these
Mathstart series - The Best Vacation Ever by Murphy, Stuart J.; Bernard Westcott, Nadine (Illustrator)
http://www.aleks.com/ month free trail if you say you are homeschooler, $25 a month, for math facts and general math
http://www.mathletics.com/ $25 a month, they will give you a week free trial if you ask. This was very motivational for My DS9. When I let him played to long he got too intense because it can involve competing. There maybe ways to set restrictions.
http://epgy.stanford.edu/courses/ some people here are getting a nice group price.$135 for school year. I'm not sure what's up for the summer. You may want to search for threads. I think they have a LA and Math combo.
see mindware.com, WE love the workbooks! I think they have some multiplication game too.
This should keep your child busy this summer.
