Physical activity is not only important for your child's health - it will help them cope with the sheer physicality of interacting with twenty children on the playground. Bumps and shoves are inevitable, so make sure your child has lots of physical play to develop gross motor skills too.
Many online math games are designed purely for entertainment and won't do much in terms of teaching your child. While these games can be fun and engaging for third graders, you want to choose games that will practice the skills being taught in school.
Look for games and puzzles that are part of an adaptive learning program. This means that the online games are well-thought out and match the same set of skills that are being taught in the third grade curriculum. The program is structured toward each individual student and fills in the gaps where the child is struggling.
While there are many preschool worksheets available, some are more useful than others in terms of versatility. There is a lot more to school readiness that just knowing the alphabet and counting to ten.