Picnic, so long as the weather holds. This can be done anywhere. Have some fun and do it at Paradeplatz, rugs and all.
The ferries on the lake are wonderfully relaxing. Picnic there. Drink wine.
Walk along the Uetliberg. Picnic. Drink some wine. Make a fire, grill lamb racks.
Arrange a private tour at the Masaola Hall at the Zoo. Swiss Jungle! Picnic.
Buy frisbees and spend the afternoon at the Lakeside, ZuriHorn, with the Rastas. You could have a picnic.
Of course, tell them to bring swimwear. (Your friends, not the Rastas)
Do a walking tour of Zurich. Search this on the Forum.
Bring them to the Talacker on Wednesday night. I guess a picnic would be OK there...