I second peacock or column. I've done column and they are pretty cool to do and am dying to find the time to do a peacock. Maybe in the next few weeks. However, I have to make a bunch of restocks first.
Oh yes, definitely do a peacock swirl! You can also do some other fun things, too, such as setting a decorative soap embed on the surface of the batter where the middle of each bar will be, or even several embeds like so:
Another fun thing I've been doing lately with my slab mold is lining the bottom with a decorative fondant mat cut-to-size, and lining the sides with heat-resistant mylar, then doing a 2-colored layered soap in it with a little bit of a swirl in the top layer, like so:
And this is what I've been doing in my slab mold when I want to do a peacock swirl, but my FO is too much of a fast mover- I split my batch up into about 90% or so scented, and about 10% unscented. I pour my fast moving scented portion into the mold first, and while it's setting up, I split the unscented portion into about 4 further portions and color each one with a different colorant, and then do a peacock swirl with them just on the top of the soap, like so (oh- I threw some soap confetti into my scented portion in case you are wondering what those colored spots are in the bottom portion):
I have seen a couple of YouTube videos from Holly at Missouri River Soap, a couple of drop swirls. I think that if you wanted a larger amount of soap a slab mold would also be perfect.
I remember watching a soaping101 video and she did a goat milk soap and did some pretty design on top. I think it was some form of Japanese art or something but it was really pretty. Look it up on YouTube!