The first tree I learned to identify, after cork oak and pines, is the London Plane. By the way, I am almost exclusively using the Kingfisher Field Guides Trees of Britain and Europe pictured in that post. I was very excited to learn the London Plane because they are everywhere, in every playground and I remember them from my childhood! I clearly remember the spiky balls and wondering if that's where walnuts come from. Well friends, it's not. This is a tall tree that is easily spotted by its scaling bark with patches of creamy white and by its globular, spiky fruits.
This took me forever and online help to identify because they are not flowering right now. When they are flowering, their purplish/blue flowers are easily spotted and fall everywhere, especially on your car. They are very common in Lisbon. And around my house, too. Luckily they still have their woody seed pods, which are tough and 2-3 inches big. They have leaves twice divided into leaflets.
Silver Birch/Vidoeiro branco/Betula pendulaI was happy to find a silver birch tree because they also remind me of my childhood neighborhood. They have beautiful smooth, white bark, they are slender and have long, green catkins (those caterpillar-like "fruits" of trees).
This is actually a shrub, and not a tree, so I didn't find it in my book. But it is so common and I've seen it so many times that I wanted to know what it was called. And now I can see the difference between shrubs and trees better.