I wear a German brand of sandal called Bama, but I don't think they're available in the US/Canada. Good to know if you have a shoe emergency overseas and need to get something reliable, though. My kids wear Columbia brand walking sandals, they have a lot of nice styles so you might be able to find something comfy but also a little stylish. Quality is good, too.
If you're planning on mainly city walking and light off-road rambles, something like a walking sandal would be fine - I've worn mine all summer long, rain or shine, all over Europe and a little in N.A. for the last three years now - I wouldn't do any real hiking in them, but they were fine for walking up Vesuvius and in the woods and hills around where we currently live in western Germany, as well as weeks of walking all over European cities and towns.
I don't think you'd need runners/sneakers at all, but if you plan on visiting clubs or going out anywhere nice, you might consider bringing along a pair of flat dressier sandals - I have a pair of Minnetonka sandals that are simple leather with some beads on, I can walk around in them all day, but they're also good enough looking to wear out in the evenings, and they take up about as much room and weight as a pair of flip flops. If I'm just going on a short trip and won't be going off road, that's all I take.