You can move Documents, Pictures, Videos, Desktop and other special folders to another disk - no problem. Just go to your C:\Users\YourUserName\Documents folder Properties, open Location tab and click Move.
However if your computer start without that external drive - Windows will re-create these special folders in default location and you will have to move them again after you connect HDD.
According Program Files - it is possible to rename or move Program Files and Program Files (x86) directories, but I just create another Program Files directory on second HDD and install very big programs there.
It is good if you have .NET installed on SSD (many programs use .NET libraries).