Frauenfeld has an absolutely enormous field on the banks of the Töss where we take our dog occasionally. There's always plenty of people there on a weekend, but everything is so spread out that you can choose to let your dog play with others, or you can just play fetch with him/her and let them run around alone away from other dogs if you choose.

Sorry for a lack of better directions, but coming from ZH on the highway past Winterthur, take the second Frauenfeld exist, go right, and you'll see it. Impossible to miss, frankly.