Sometimes, it's hard to start without an adventure idea, however. And sometimes, you get an idea that's too cool not to use.
In the vein of the second one, I present Slaadi Poker. Suitable for any edition of D&D, and some minor twiddling can suit it to most fantastic settings. It is, however, a bit... silly, and might not suit all campaigns.
You fell through a portal. Or, you touched that funny-looking statue you know you weren't meant to touch. Or you were kidnapped. How you got here isn't important - what is important is that you're now in the presense of four beings of immense power - and your only way out is to keep them entertained.
Somehow, the players have ended up in Limbo - a roiling chaos of pure thought-stuff, the Primordial Chaos, if you will. And with them, there are creatures of pure madness - the Slaad. You've just caught them enjoying some down time from their overwhelming madness, and they're playing poker.
Slaadi Poker is... an odd game. The rules are never the same, round to round, and freqently new ones are created on the spot. Never to make the player win, of course - just to keep things interesting to creatures who couldn't care less about Order and Rules.
The players can try to appeal to the Slaad, but it would take a lot of patience to get through to them, or even put across a simple point. They can try and figure out how to get home, but a surprisingly lucid Slaad offers to send them back - if they play a hand or two.
In this game, "turns" don't matter - it's who can get to play first. Calculate initiative in the usual manner, to figure out who goes when. Players can make Hard checks to discern the rules - skills involving Planar Knowledge, Gambling, or anything else the DM decides is appropriate. Of course, they will need to make these each round to keep up with the game.
The easiest option, which can be suggested by the DM after Hard checks against skills involving Bluff, Charisma, or Intelligence (Sense Motive would be perfect for 3.5) is to simply start making up extra rules instead of playing. These can be as simple as "No, it's his turn next (in which case, re-shuffle the initiative order to fit), or as obscure as "But you can't play that card - it's Fat Sally rules, thoise can only be played after three sevens in a row, on a Friday."
The Slaad will either play a card, or play one with an extra rules rider, or will simply add a new rule to the game (or discard an old one). Again, be creative - it can be a rule which actually affects the game, or a general statement regarding which ruleset they're using. In some cases, the rule can come from a completely different game (such as having one particular Slaad occasionally chip in with "Checkmate.")
The game can't really be won, but after a while, the Slaad (or the players) may get bored, and declare one player the winner. Or, of course, one of the players might suggest a rule that can, in some way, lead to them winning - a good example would be to declare a tie because they both played a card of the same suit/colour/shape, then make a challenge to break it - so, out of the player and one of the Slaad, who can guess the outcome of a coin-toss, or who can draw the higher card from a deck, etc.
A suggested prop for players would be a deck of Slaadi cards - random shapes, colour, materials, and pictoral representations, to give the game more "feel". Also allows them to actively "chip in" instead of making it an entirely static skill-based encounter.
If you wanted, this could even be run as a free-form RP section - no dice (unless the poker game calls for them), just pure banter. This works better with established groups, as everyone will feel more comfortable throwing in ideas and really taking the chance to role-play the situation out. Doing this can also reduce the game to a Mornington Crescent-style game, where no cards are played and only the ruleset is discussed - even better! Gives the players a chance to shine as they create wild and wonderful rules - just make sure no-one's being left behind (unless it would be entirely in character for them to simply remain bemused as people argue over whether or not they're using the Floppy Bunny card under the Sacred Transvestite ruleset).
- The player has some connection to the Slaad/Chaos - maybe a planar tourist has seen the game played before, or (in the case of 3.5, the Chaond race) has Slaadi blood. Maybe they're just crazy enough to tap into the Slaadi mindset. Bonuses as appropriate.
- The player is an expert bluffer, or uses spells to make him more charismatic - grant appropriate bonuses when these are well-explained.
- The rules the players come up with are particularly funny - after all, this is a humorous encounter.
- The Slaad decide to change the rules to something totally different. Maybe it's become a dance-off, done as a combat encounter, maybe one of the Slaad "cheats" and the game decends into a bar brawl, whatever - make a complication based on what the players might enjoy more!