I was once a daily running fanatic who wouldn't skip 5 miles a day despite heavy travel and know how hard it can be to find uninterrupted ground in cities.
Off the top of my head, I've done laps around the Superdome (NO) & Georgia Dome (ATL) - both provided a bit of shade in 90+ degree summer heat.
In NO, you can run along the trolley routes (good tip from Runners World), mostly grass
Tampa, the long bridge to Clearwater (Rt 60) very near the airport, a cluster of hotels on Rocky Point
Tampa, the otherside, Bayshore Blvd, a long paved waterfront path, used to Rollerblade that one too
Portland, for sure, paths along the river and cross a bridge and another back
I'm sure I got creative in other areas, will edit when I recall