The most boring part in the game is the end game. You are able to see rather soon that you are going to loose.
An surrender button would be a quick solution but far more interesting would be a changed gameplay for us who love boardgames.
The trick is to keep the simpleness of the game but making it harder to see who wins in the end.
Another problem that Risk always has is the player elimination and the zero chance to come into the match again when falling behind.
The solution here is a little harder to implement and still keep the risk feeling.
I rather would like to mail my ideas instead of writing them here if it is possible for you.