Dungeon of the Ghost Tower (Dungeon # 182, September 2010) -- which is the follow-up to the Essential Red Box 'choose your own adventure' character generation. It's a nice little self-contained dungeon with some puzzles, some (fairly easy) combat, and a suitable boss monster. Probably too much to get through in one session, but easy enough to cut some content by walling off certain areas.
I can recommend a variety of Living Forgotten Realms (LFR) adventures; The Radiant Vessel of Thesk which was just recommended is indeed a good one. All of these are designed to be completed in one session (4-5 hours). Obviously, they all take place in the Realms but most of them could be plopped down wherever.
[sblock=LFR level 1 recommendations]
CORE1-3 Sense of Wonder - PCs are literally summoned by a cleric of Gond (gnomish god of crafts / artifice) to investigate a lost temple that has surfaced. Has a pretty fun puzzle, if you like that sort of thing.
CORE1-13 The Fate of Camp 13 - Not one, not two, but three different factions the PCs can ally with or oppose re: the titular camp. Could be some lessons here for younger players on hearing all sides of a story before jumping to conclusions.
CORM1-1 The Black Knight of Arabel -- Combats are *way* too easy and skill challenge is ultra cheesy, but the overall tone of this adventure is superb. Good Guy knight is in disguise as Bad Guy (so, again, younger players could learn to do some investigation before the bashing) for... reasons. Can easily swap in updated monsters from Essentials to make the combats more interesting.
(Hmm, years 3-4 were kind of a wasteland, nothing I'd recommend there.)
SPEC5-1 Morthac's Mansion -- Officially a 'special' which is more difficult (i.e. higher XP budget used to build combats), so you will have to judge how this will play with your group. Not really tied to anything critical so you can reflavor the reason the PCs are on the mission to the mansion. Bonus (or drawback), all takes place on the same map, from one of the poster map packs. Features some exploration / puzzle elements which I enjoyed.
SPEC5-3 What Distant Skies -- See above re: 'special' designation. This one actually is pretty difficult. Tied into a convoluted storyline involving sky pirates, so you'll have to do some work to extract out the relevant parts. Full disclosure: I am listed a co-author on this (although Chad Brown did 90% of the work).[/sblock]