Create a new bookmark, and use this as the URL:
javascript:(function(kws){function Rnd(w) { return parseInt(Math.random() * (w + 1));} ;var url='http://delicious.com/tag/'+kws[Rnd(kws.length)-1]+'?min=10&random='+Rnd(8); location.href = url;}(['chrome','google','computers','youtube','history','internet','technology']))
You can add tags where you see
['chrome','google','computers','youtube','history','internet','technology'] to broaden the categories this scriptlet will Stumble from.
Name your new StumbleUpon Bookmarklet "Stumble!" or something of the like, and off you go!