As far as programs are concerned, there's no way to change the default installation directory unless you relocate Program Files entirely. That's tied in to the ProgramFilesDir environment setting, so an application is always going to try to default to installing in wherever ProgramFilesDir points (and if it doesn't point to Program Files, that's very bad).
Your options basically boil down to deciding what you want to keep on your SSD and what you don't. Relocating your special user folders (My Documents/Music/Movies) is trivial; relocating the entire user directory is harder. You could save a ton of space by relocating Program Files, but then parts of your OS is not on the SSD, which is counterproductive from a performance standpoint.