I have a overnight in Helsinki in July. Really only 9am to 12 noon. I will be downtown. Any suggestions?
You'd have to be careful about the ferry times, but Suomenlinna fortress in the harbor is wonderful during a summer morning. You'd get 1.5-2 hours to explore the islands on foot. The islands themselves are pretty interesting, but you also get amazing views of Helsinki on one side and the Gulf of Finland on the other. Enjoy your time in Helsinki. -TJ
At a minimum, check out the harbor market. Might be too early for lunch, but they serve some interesting food there.
Plenty of time to walk around near the harbor and Cathedral. Have breakfast in the glass coffee house in the middle of the park (just down from the harbor, you can't miss it). Walk through the market hall. Visit the cathedral and maybe stop in a few shops. If possible, have an early lunch. My favorite is one of the food vendors in the old market hall - quick and inexpensive.
Rooftop of Hotel Torni for great views of the whole city and a few pics. Market for quick lunch, may I suggest Reindeer if you are not a vegetarian? Visit main cathedrals: Helsingin tuomiokirkko and Uspenski
Maybe you have time for a quick visit of Finnish national museum.