When it comes to kimono, there is not much leeway for jewelry. Necklaces are typically hidden by the collar, brooches would potentially damage the fabrics. Subtle bracelets, a watch, and a ring or two are typically considered acceptable, but if you want big, flashy, statement pieces, the place to go is for obi decorations.
The most common sort of these would be obidome, brooches worn on the obijime. I only have one “real” obidome, and have already written about it. Obidome are possibly one of the most expensive items in a kimono wardrobe, often made with pearls, diamonds, and other rare gems. World-renowned Japanese Jewelry designers such as Mikimoto have been known to make them, and the prices typically reflect this. It's also rather rare that they trickle down into the secondary Western market, which helps to drive the prices through the roof, even for pieces that don't have gems or precious metals in them.
However, there's no reason they (like many other kimono accessories) cannot be improvised! Here are a few easily attainable items I've been known to use in lieu of true obidome.
Vintage Agate Brooch
This was a brooch belonging to my grandmother. It's a large oval chunk of agate with a filigree frame. I've got to be careful about this one, since I do pin it to the obijime, but as long as I'm delicate, there's no permanent damage.
Vintage Leaf Scarf Clip
This is a vintage scarf clip, typically used to hold a silk scarf around your neck without damaging it. The clip mechanism is great for snapping onto a wide, flat obijime. I've been meaning to buy more of these, as they are cute, versatile, and very easy to use.
Handmade brass belt buckle
This is a brass belt buckle my grandmother had made when she was younger. The designs on it remind me of typical Japanese motifs, maybe plum and chrysanthemum. It fits well over the knot of a rounded obijime, and looks like it was meant to go there.
I honestly have no idea what these are intended for. They were on leather cords when I bought them, so I suppose they could be used as necklaces? In any case, they fit well over smaller obijime and add a lovely bit of sparkle to a casual outfit. Unfortunately, they're glass and they do dangle down so there's always the risk of clipping them against a table or counter, so I don't wear these out much.
The other alternative I'm rather fond of are netsuke. Traditionally, netsuke were worn only by men, and used to hold inro (medecine/tobacco pouches) in place on the obi. It is becoming more common for women to wear them though, particularly “cute” ones, with casual outfits. I have a fascination with all things marine, cephalopods in particular. I've never seen an octopus or squid on a kimono, however they do show up on netsuke with some regularity, so this is a way for me to inject a little bit of my obsession into my outfits.
Catfish and Ball Netsuke
This is slightly older than the previous two, made of a darker, heavier wood. It also has an ivory inset on the other side with an artist's mark stamped into it. I'm not sure what the ball is supposed to represent, but I think it's cute.
Tiny Octopus Netsuke
This was a surprise gift from Christy, and it made me squeal with glee. It's another octopus, but it's absolutely tiny and delicate and ridiculously adorable. I love how it looks sort of like a grumpy old man. I've not worn it yet, but I can't wait to!