I too have a son starting Kindergarten this upcoming year. I did not send him to preschool, as he was far better off learning with me and to allow him time with children of his own age, he has participated in many activities. I not only wanted him to be around children his age, but also felt it necessary that he learn how to follow instruction and to be attentive, so he played tee-ball for 2 seasons and attends "library school" once a week, where a retired teacher who is also on the board of education, holds a themed class, reads books, crafts and it is done in a classroom environment with other children his age that are learning at home. We meet once a week.
He had his Kindergarten screening test done a few weeks ago and we later had registration day, where was able to explore the school and meat teachers, other children that he will be with, and just basically go over the curriculum and procedures. We will have orientation later, which I highly recommend you attend with your child. When my daughter started, we attended and it was 1+ hours spent with the Kindergarten teachers and concerned parents of other children and it gave the children time to become acquainted. We were told what would be expected and given examples of what to do over the summer months to prepare for the journey.
Back to what to do over summer, our plans are: we will have "class" a bit each day. I do this each summer with both children. For my older daughter, we will work on subjects that was challenging for her and my son will continue to practice his writing skills. He counts forwards and backwards to 100 and has begun to do simple addition and subtraction, so I plan on working with this skill. He recognizes all letters and order of the alphabet and sounds they make, but I will be working with him on vowel sounds, such as long and short and continue with prefixes and suffixes. He reads but has limitations, so we will continue to develop his word recognition and focus on all sight words. We participate in the Summer Reading Program at the library. These are so much fun. I attend 2 different libraries in the summer months because the two offer such different events. Our primary library teaches themed subjects with crafts and there is a reading list with rewards for participation and accomplishments as well as a great cook-out when it ends on rewards day. The other library, doesn't "teach" or have crafts or activities, but they have someone come in to perform for the children, such as a magician, musician, storyteller, etc. They will talk to the children about important subjects such as safety, bullying, manners, behavior, etc. So check with your local libraries to see what is available.
We go to the park, take short "field trips" visit museums, nature walks, etc., have picnics, participate in organized events-such as swimming lessons, ball practice, exercising, community events, workshops, cookouts, etc.
We explore, have fun, learn, enjoy the outdoors as much as possible and of course read, take a vacation and explore other places, but never would I send my children away to a camp or leave them at their own will for "downtime" or to learn on their own with "activity books". They need guidance and need to keep moving. Play dates are fine, but with all the involvement in local activities, they probably won't want one because they get fulfilled and prefer quality time with you in their "free time".
Also, do something special just for him to mark this special time in his life. Relax, enjoy your time together, learn and you will both get off to a wonderful start in the new school year.
I've included some links of sites that we use besides education.com. Starfall is a wonderful site that Kindergarten students use on their own at my school, so you may want to check it out.
Best Wishes!!!!