Tomb of horrors. Extra people won't help you.
Or Keep on the Borderlands - the first time I ran that, it was one on one, though I had two characters. A fighter (Boris) and a wizard (Altalazar) - and the fighter fell down a pit trap in the first cave and died in the first five minutes. LOL.
Later, when I started playing regularly in College, the first two characters I made (starting from scratch again) were Boris and Altalazar. Fortunately, these two incarnations lasted much longer - I think by the time I stopped with 1E, they had a nation, an army, many followers, and were F15 and W16 respectively. I must have made a hundred custom spells, too, along with a rather extensive Mage's guild.
Ok, now I've TOTALLY gotten off topic.
Dungeon Crawls have a certain simplicity for one on one play.