Roundhay Park is one of the largest city parks in Europe and one of Leeds' most genuinely lovely spaces, the lakes, the formal gardens, the Canal Garden, and the woodland walks cover enough variety to make a full afternoon out of it. It's where Leeds residents come to remember why they live here.
The café by the upper lake is worth a stop; the Tropical World attraction on the grounds is excellent for a rainy day alternative. Entirely free, entirely worth it.