You should have a look at https://1shotadventures.com/ ! They have a variety of free one-shot stand-alone adventures for all kinds of genres, with pregenerated characters for each. It's all 4th edition, but the conversion between versions shouldn't be bad (and like GoldSeal said, 4Ed Lite is free!)
Right in the book, there's a starter solo adventure called "All In A Night's Work". For a whole table, I would recommend converting free oneshot 4e adventures like the other commenter.