We went to Yelagin for the first but far from last time with some friends last weekend. What a treat! It is a car-free park, full of trees and grassy areas and a petting zoo, cafes and fresh air. Alex loved picking weeds to feed the sheep and next time we'll remember to bring bread for the birds. We even ran into some new friends who live a few blocks away ... and we're totally envious of such a great location (although of course we also like being walking distance to downtown Piter, so I guess you win some, lose some).
Here's the wiki entry, which does not do justice to the experience.