I agree with your assessment on both styles of play. Campaigns can be very rich and rewarding in the long run, but they are definitely harder to pull off. It's much more work for the game master and it's harder for the group to consistently schedule game nights.
One night adventures can be a lot of fun and it's nice to take a break from a campaign every now and then (or maybe between campaigns) and spice things up with a single night adventure. One thing I love about them is that you can get very experimental and try things that you never would have tried in a campaign. It's sort of like a short story versus or novel... you can be much riskier. You can also do some things that simply wouldn't work for an extended period of time.
Here are some examples for ideas I've had for single-shot adventures...
1. One idea that I've tried a couple of times is that the characters wake up with no memories and no identity. They wake up in some kind of alchemy lab or wizard's spell chamber or maybe even something more exotic (biological egg sacs?). Anyway, they wake up and they have no memories at all, no clue who they are and no idea where they are. And they immediately realize they are in a dangerous place (maybe some kind of animal starts to try to break down the door to get into the chamber where they are). I think this could be a lot of fun... especially for the players. They are thrown immediately into an action scene with no weapons and no resources. They have
no character sheets! Give them a blank piece of paper! As they gain information about themselves, they can right down stats on their character sheet. But initially, it's blank. Not even a name. For this idea to work, the game master has to take on a lot of extra work. The players don't even know if they can cast spells yet. As the adventure proceeds, they have to learn to trust one another and they also start to piece together clues about who they are and what's going on. This idea would actually work in any genre, any game system. I've run it once in a fantasy setting and once in a sci fi setting. Coincidentally, I saw a sci fi movie last year where this idea was implemented. It was called Pandorum starring Dennis Quaid and Ben Foster. In that movie, the two main characters wake up from cryogenic hibernation on a starship - they have no memories and no idea what their mission is. Similar idea... anyway, I think this one could be a lot of fun. Of course, this is custom made for a single-shot adventure. By the end of the adventure, they've figured out who they are, what's going on and they either complete a mission or escape from wherever it is.
2. Another idea is to put the adventure within a very simple framework with a very simple goal - escape from a prison or navigate a deadly maze while being pursued by an enemy or perhaps simply survive a dragon attack. During one single shot adventure, I had the players acting as bodyguards for a magistrate in a city who was being targeted for assassination. Their mission was to keep the NPC alive at all costs. They sealed themselves up in the magistrate's fortified mansion and erected all kinds of defenses while wave upon wave of bad guards assaulted the place. They had to survive the night until help arrived in the morning.
3. Another thing you can do is go all out with something you normally don't do in a regular campaign. One idea would be to let the players roll up very powerful, very high level characters and let them arm themselves to the teeth with every imaginable weapon and magic item... whatever they want. Then give them a short mission with a suicidal goal. Something like rescue the princess from the Demon Lord's palace in the bottom plane of Hell. Put them up against hordes of demons and see if they can survive.
4. Shipwrecks are good for single shot adventures. Have them shipwreck on a small island which isn't on the map. And fill the island with all kinds of adventures.
There's a million ideas for single shot adventures. I actually do love that type of adventure, but curiously most of the stuff I run ends up being campaign length... dang, I need to another one of these!
