While it won't be exactly the same as a wall or ceiling† bounce, you can move the flash off camera and make it significantly bigger without moving it off camera. Something like the Lumiquest Big Bounce will make the light source quite a bit bigger and softer, while elevating the light source enough that pictures taken at typical portrait distances will have some modelling.

I say "something like" because the actual Lumiquest unit is kind of expensive and really pretty heavy—it will easily tilt a flash that doesn't have a tilt lock, and it puts a lot of stress on the flash foot and hot shoe. It's actually something you can easily create yourself out of foamcore or poster board and attach to the flash with a rubber band for a couple of bucks, and while it won't be nearly as durable, it will be both much cheaper and much lighter.

Another alternative is to use a flash bracket and a softbox. I'm speaking of something in the Westcott Mini Apollo or the Photoflex LightDome XS range. At about 12"x16" to 16"" square, they're not exactly a seven-foot octobank, but they are huge compared to a bare speedlight, and a decent (and not too expensive) flash bracket will get them far enough off camera to give some modelling. (And there are cheap knock-offs available if the price hurts too much.)

If you don't have an assistant and are comfortable shooting one-handed, there's also an arrangement like the Lastolite Brolly Grip to consider. Again, the "real deal" might be a bit expensive for what it is, and you can assemble other people's parts or do a DIY version of the actual grip and just pay for a cheap mini umbrella. (In fact, you may find a suitable translucent white child's umbrella at your local deep-discount emporium/dollar store.) It'll work best with an SC-29, which has a 1/4"-20 nut on the bottom, so anything you can stick a 1/4"-20 screw out of will work as a handle.

Those are just a few options to chew over if you need to be mobile. If you're shooting in a fixed location, you've always got the option of bringing your own wall or ceiling—a large pop-up reflector (or a sheet of foamcore) on a bracket on a light stand (again, that doesn't have to mean using the official, photo-certified version) or a V-flat is always an option.

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† Does it bother anyone else that ceiling is spelled/spelt the way it is? We stole it from French before fitting the ing attachment to indicate its smallness compared to the real sky, and there it's spelled with an ie rather than an ei. That's probably something we Canadians are a little more sensitive to than most, I suppose.