The Travelling Salesman Problem, but for transit and walking tours. There are a few online tools that generate the quickest route between points on a map, but they assume driving. I'd like one for transit, that, OK, helps you get your errands done around town, that would be cool. I'd also like one for tourists: given a set of lat/lon coordinates, give me the fastest transit routes between each with some time in between each point (say a half an hour, or a random amount of time between 30 minutes and 60 minutes) to run your errands. For tourists, randomly generate a walking tour like Walks.io does for tours around rail stations in the UK.