I visit my grandparents on sundays and other days/ on other days I like to rest at home.
You can use either one, but make sure you capitalize "Sundays."
(You can also say "I like to stay at home and rest.")
Six days of rest for one day of visiting? They must be very active grandparents to wear you out so much!
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.