I will be in Amsterdam and other Baltic areas May 9 through May 20. I am looking for advise concerning taking rain gear and general packing suggestions. Will tennis shoes be advisable or will the possibility of rain dictate another shoe choice?
Amsterdam doesn't have a wet season. Rain is distributed throughout the year. It is rare that you will meet heavy storms, but also rare you'd meet 10 days of straight bright sunshine.
Always pack for the possibility of light rain.
I have been in AMS both in March and in August, and there were equal amounts of rain. The skies are cloudy, beautiful, it rains on and off, and then it stops for a while, and so on. It won't get in the way of any plans, and is a normal part of everyday life there.
I would stick with tennis shoes (not rubber boots) but bring a rain jacket. Small umbrellas can be purchased there if you decide not to bring one. Forecasts from www.weather.com and other sources can help refine the plan as your travel date gets closer.