I'm kind of in the "semi-random" school of DMing, in that if I set up a random encounter table for an adventure, each possibility will have some thought behind the wheres and whys, rather than the aforementioned '1d4 manticores'. Not always - some will be because there are a high number of Whosits in the area and so walking around for a few days you are bound to see some. And not all are things to fight, of course. Merchant caravans, Lone Survivors etc. are all fun.
Having said that...
I once ran a little solo campaign for myself (so sad

!) using only the random tables in the DMG (including the old random dungeon stuff for indoors). *That* threw up some interesting situation, especially the time the characters were faced with rescuing a unicorn from four ogres, and they were 2nd level. I posted that one on rec.games.frp as a tactical problem. I think Entangle was used to good effect....
Further, random tables can be expanded beyond what you meet to include what they are doing. RuneQuest Cities is a good supplement for this, and 2nd Ed. Jorune had a similar thing. In RQC (which can be expanded beyond Cities, it's just that most of the encounters are human types), you might roll and encounter with... Barbarians. So then roll on a sub-table and the Barbarians are... Drunk and Looking for Fight, or... Wanting to Hire Adventurers to help them (which might lead to a further set of sub tables - very useful for adventures on the fly which would give you an adventure like ... Doing What? - Steal. To What? A Book. From Where? A Graveyard. Eh? Well, perhaps there's..er..a Lich involved. Etc.
Takes a bit of work and rolling, doesn't always give logical results, but they are pretty useful. I've even used them for Babylon 5 adventures, with a bit of tweaking. Barbarians might become, say, Drazi, for example, and the book becomes a datacrystal.