I don't know how busy you routinely are, but here are some ideas.
I go to the gym regularly, leaving the kids in the childcare area for a couple of hours is similar to spending time at preschool.
I'm not a SAHM but I am a teacher who had summers with her kids when they were small. I took advantage of all the free stuff I could -- library story hours, museum visits, going to the park, the pool, the skating rink, things like that. I liked the days to have a predicable "shape," that is we would usually do household things early in the morning, a couple of times a week have something in the mornings, like library story hour, go eat lunch in the park or something, then come home for some down time. I think most three year olds and a lot of five year olds do benefit from a rest time in the afternoon.
I don't know if you are Christian or not, but I found my kids at four and five usually enjoyed taking advantage of the Vacation Bible Schools that are offered all summer long in the churches in our community. We usually did two of them each summer.
How could I forget the library, lol?
BreakfastMornings (when it's cooler): park, backyard, zoo, stroller/bike ride, sidewalk chalk, anything outdoorsLunchQuiet time/napAfternoons (when it's hot): library, errands, aquarium, childrens museum, indoor playplacesDinnerBath/brush/books/bed
Check your newspaper for any upcoming local events that might be interesting to their age group.
What would men be without women? Scarce, sir, mighty scarce.
when my DDs were 3 and 5, summers weren't too different than what we did on days they weren't in ( p/t ) preschool.
i ddi not sign my kids up for VBS until after they started school, the end of kinder or 1st grade.