A great place to go for a walk, run or jog is Backbay.  There is a pedestrian walkway that goes all the way around the bay so you can exercise safely and get a great view.  Parking is available on the Preble Street extension across the street from the entrance to Hannaford supermarket. 

Mackworth Island, actually located in Falmouth, is only a few minutes drive out of Portland, and is great place to go for an easy hike.  Drive across the little bridge, tell the person at the booth that you want to go for a hike, and you can park for free in a small lot to the right.  The island is very woodsy and hilly, and offers some terrific views.  Walking once around the hiking trail loop takes around a half hour if you don't stop for too long to take in the views.  Along the trail, there is a section of fairy houses, where people build little houses out of twigs, branches, leaves, or whatever they find on the ground.  The only rule is to not destroy any of the other houses.  The fairy houses are a little reminiscent of the Blair Witch Project, but overall they are cute in a woodland pixie-like way.  There is also a school for the deaf on the island.

Another option is walking the trail by the Eastern Promenade beach.  I'm not sure how far it goes, but I think a few miles in both directions at least.

 As for sports, don't miss the Portland Sea Dogs minor league baseball, only in the summer season.  Tickets are cheap, around $6 a game, and it's all the fun of a major league game with mascots and ballpark eats.