Ways to enter special characters? Is Alt+Numpad possible?

You can use compose sequences which are more usable than the Alt+num method in Windows. For more see https://help.ubuntu.com/community/GtkComposeTable

For example, Compose + t + m = ™

Ctrl + Shift + U works fine (under XUbuntu 15.10 and most likely under many other X11-based versions). In contrast to the compose-key method it allows the input of any UTF-8 character. E.g., Ctrl + Shift + U 263a +Space yields ☺.

Of course, I use the compose-key method whenever applicable.