Wear good walking shoes and check the weather forecast on the day of your visit, be sure to dress appropriately. Swedish weather is notoriously unpredictable in all seasons.
Weekends tend to be very busy at Rosendals, especially in good weather. If you are visiting with small kids it might be best to go on a weekday.
This itinerary is best suited for summer, but can also be great in the spring or fall in good weather.