Catching Pokemon and clobbering gyms takes a lot of balls and potions. What's the fastest way to refuel your bag? I've been looking around at different circuits in town to maximize the item grind. My main goals: get the most items per hour with something central, easily walkable, and mostly optimized to register a good amount of distance.
Here's the best walking circuit I've found, which will get you 13-15 spins per brisk 5 minute loop (up to 180 spins per hour, for almost 600 items). Start at Main and Washington, head south down main to Liberty, turn left and walk to 5th, then turn left and walk to Washington, then back to the start. The stops at Washington and 5th are a little tricky, but I'm able to reliably hit all three without leaving the corner. This also has the advantage of being fairly efficient for distance tracking, since there are a lot of straight lines.
Here's a circuit on central campus (Diag / Ingalls Mall) which will get you 26 stops every 5 minutes of easy biking (31 with the green add-on paths). That's a whopping 312-372 stops per hour, getting you 1,000-1,200 items. It's a little over 1km, there are minimal road crossings, and as long as you don't hit a class change, it's very easy to get around the students. There are also tons of natural spawns for you to grab along the way. The only downside to this path is that you'll be picking up so many items that you'll be dumping stuff by the hundreds to free up bag space! It's also shaped like a bird-dog, which is sorta cool, I guess.
Car grinding sucks. It can be dangerous, it wastes gas, and it kills the whole point of the game: get out and get some exercise and fresh air. That said, I know it's pretty common, so I'll stop preaching and share what's probably one of the easier, safer, and more productive circuits in the area: UM central campus. There's a giant caveat with this: don't try this during peak college traffic times!
Let me know if you've got your own favorite item grind circuit!