I actually love stumbleupon! It's what brought me to reddit in the first place. I would usually find new subreddits by browsing the comments section. A lot of people would link to some relevant subreddit. Another great way is to follow any links in the side bar of a subreddit (or to browse related tags in the case of hubski).
If you want to contribute new content I would suggest following more blogs and news websites. Blogs especially seem to be content creators.