If you want to go beyond the usual, the rainbow you describe to your players probably has nothing to do with rain or leprechauns. You can roll randomly for a result, use the handy number provided with each entry to figure out your result on a d12. You can also pick the one that works for the area in which your characters currently linger.

d12. Curiosity

1. The rainbow ends in a mirror the size of a gnome’s head. You can see someone inside the mirror waving frantically to be let out.
2. The mushroom that overwhelms the area with its vastness sits at the end of the rainbow, hungrily eating it and growing even larger.
3. It’s interesting, but when you get to the end of the rainbow, you see that it’s made of some sort of thin substance that floats forward and clings to you permanently. Well, maybe if you changed your shape permanently, it wouldn’t cling, but are you willing to find out?
4. You weren’t aware that this many fae folk existed in all of creation. Wow.
5. Apparently, the rainbow signifies the meeting and location of unicorns for various Needful Things. You probably don’t want to know what they are.
6. “Yay! ADVENTURERS!!!!!” shouts out a tiny little kobold. “You’ll help me, right? I sent up the right signal, right?” The tiny kobold gives you an adorable (for a kobold) wide-eyed smile.
7. The black hole at the end of the rainbow looks just big enough to fit you if you decide to jump in.
8. It took a lot of effort to make a prism that big.
9. Since the beginning of time, rainbows have been seen as good omens. You will crush all hope that’s left in the world with the reality of what causes rainbows — will you share your potentially forbidden knowledge, and what exactly is it? (GMs should allow each player at the table to choose one aspect of what they see at the rainbow’s end that MIGHT spell the world’s destruction. And now you have potential adventure fodder! Yay! Because who can let the world’s end ride without facing it down and stopping it?)
10. It took a lot of “food” to grow a gelatinous cube that big.
11. A piece of parchment states the following: “You fell for this ‘follow the rainbow’ trick, did you? Well, how convenient! Enjoy my friends—they really want to play with you, or so they say.” (The signature comes from the most appropriate villain in your current game.)
12. You’re very confused. Are you looking at you? A bunch of yous? How many yous can there be in that amount of space? And you recognize the when of the nearest you! You must warn this you! If you told it about this one thing, you wouldn’t take that negative consequence from it! But, just as you open your mouth to say something, the next nearest you starts to say, “You should know that—” and then pops out of existence like a soap bubble.