I like to relax on my non-park days. Water parks are not relaxing to me. That counts as a park day. I spend my non-park days at our resort pool. Maybe dinner/lunch in Disney Springs with a touch of window shopping to and from dinner.
If the plan is to hit a park that evening anyways, I'd just do more park time. Not a fan of the water parks and DS is great for dinner in the evening but not something I would plan a day or half day around.
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