Upgrading buildings earns you Event Points. You can do as many of those as you want and complete the track without spending any money.
That is a great solution and one I’d not thought of - thanks so much for the suggestion! At my level, however, I think it would cause me trouble because it would level me up too quickly and I don’t have the storage capacity and infrastructure to cope. I found another city building game that I think will suit me better, but if I ever decide to return to SimCity Buildit, I will try your strategy for sure.
