Swimming has several... swim together, splash each other, teach to swim (for an adult/child together)
There;s several others between adults/kids such as teach about manners, teach flashcards (tots), play, etc.
Also snowball fights and building snowpals, building sandcastles etc. )
There's also several you can do that DO require items but the items are available free around the different lots, so not sure how you'd count those (playing horseshoes/campfires, even group projects are given out for free every Monday)