When it comes to plot, sometimes you just don’t want to do it yourself. For those times, there are many options.
You could let a friend do the plot for you, and hope it comes out well.
Alternatively, you could go for one of many random methods and see what you get. It’ll probably be odd and a little bit funny, and it will also be up to you to flesh it out in most cases. They’ll definitely plant the seeds for good ideas though.
First up, there’s page 44-45 of the D&D Dungeon Master’s Guide, which has a table of 100 random adventures. This is the method my friend running a completely random game from first level is using, and it’s led to some hilarity. The campaign began with toads raining from the sky. Hard to beat.
If that’s not your cup of tea, perhaps you could try The Grand List of Console Roleplaying Game Clichés. Sure, it’s about console rpgs, but there’s no reason this couldn’t be used to generate new adventures for tabletop rpgs either. You might need to be creative to adapt some of the entries for use with tabletop rpgs though. Also, for random, either use the first hundred entries and roll d%, or get a random number generator and select between 1 and 192.
If you do any writing, take every story you’ve written, and assign a number. Roll an appropriate die or use a random number generator. Use the plot from the story you roll. You can do this with books you’ve read too. You’ll know the basic world pretty well too, since you either wrote it, or you’ve at least read and enjoyed it. Another advantage is that the plots are likely to be more detailed than with charts and the like, so less work for you. It is a bit more work to get everything assigned so you can randomly generate the plot though, since it’s not organized nicely into a chart or numbered already.
Alternatively, you could use a random adventure generator. Just one click and you have an instant plot. You’ll still have to flesh it out, but it will help get your juices flowing. It also doesn’t require rolling dice and scanning a table, just click and read. If you don’t have a computer with you when you plan adventures though, this won’t be very helpful.
Aside from the random adventure generator, you could also scan the lists for ideas that strike your fancy rather than doing random generation. It all depends on how you want to use them.
Hopefully, after reading through all of this, you have some ways to get your random adventures going quickly.