I am a little late to the party but as a European I have a few « recommandations » :
To book train tickets, I think Trainline is good because it regroup most of the European railway companies. It can be expensive tho.
As most of big European cities are well connected transport-wise you can change your plans if the weather is too bad up north you can go down south… Italia and Spain are great to visit if you’re in need of good weather.
Considering security questions, it depends on where you are… as you already know. But for example Germany, Switzerland or Sweden are considered safer than France, Spain or Italia. Never had any problem leaving my stuff while skating in other countries in Europe, I got a bag stolen at a skatepark once… the skatepark 5 minutes away from home in Paris haha ! Also there are « safes » in most of train stations, maybe check how it cost if you want to visit a city and skate it can be cool to put your valuable or unnecessary stuff in it for the day.
Speaking about Paris it’s a cool place, even if I am a little bored about it since I live here since 12 years, you can enjoy a day or two here. Yes we have rude waiters and lot of cigarette smokers, but there is are a lot of sight seeing to do if you are interested. Also there is a lot of spots.
Every suggestion I’ve read here is cool. I had the chance to travel a lot with my dad in Europe when I was a kid / teenager (my mom passed away when I was 8 so travelling with my old man was our way of escaping that) so i can say I brought my board to Ireland, UK, Belgium, Germany, Switzerland, Italy, Spain, Denmark, Sweden, and all the way up to the Polar circle in Norway (didn’t skate that much). Wish i did more Eastern Europe tho.
I hope you’ll enjoy your trip. What I like in Europe is that you take the (high speed) train, you fell asleep for one hour and when you wake up, the landscape has changed even within a country. Lots of cultural and geographic differences within a quite tight space and yet we have a lot of common points and shared history.