One that is a delightful self-sower and has a very pretty low mounding growth form is Geranium sanguineum 'Max Frei', for a magenta bloom. These can be studded with flowers in spring.
Another that has begun to pop up self-sown in a very restrained manner is also covered with blooms in spring, and has a gloss or sheen to the delicate leaves, Geranium nodosum, and a light purple flower-
Some other geraniums perform well but I haven't noticed them self-sowing. Geranium 'Ann Folkard' has low-growing foliage that trails several feet away and has a few scattered flowers for most of the bloom season. The flowers have an intense eye-pleasing magenta color hard to photograph, more like the flower on the right, and a sheen to the petals.
One that increases by making clumps and stays very low is Geranium cantabrigiense 'Karmina', and has a long bloom season. It is an excellent ground cover, easy to divide. There is a little Herb Robert growing with it. Other cultivars have different flower colors.
Another photo of a charming mix of G. sanguineums and a dark purple one possibly G. himalayense.
One that I grew several years but did not return finally that was a favorite was G. incanum 'Frances Grate', AKA Silver Sugar Plum, which is an appropriate name, with magenta flowers and incredible finely cut lacey silver foliage.
These geraniums have been hardy in my zone 8 (sometimes down to zone 6 temps) and some are hardy to at least zone 5, check the cultivar for hardiness in your zone.
Happy Summer, Hannah