No, the idea is not to do anything useful. It is more of a show off of what was during this age. Just showing the old UI and some applications is cool enough for ... a retro geek I suppose. The idea is to demonstrate something for a few a minutes.
Did you have specific applications in mind? Because... yeah, I gotta be frank, I can't think of anything intrinsically interesting about this era of Sun workstation. Granted I'm at a disadvantage here because back when I was stuck actually interacting with these Solaris boxes regularly it was almost entirely as servers, but I did a stint for a few months at a company that issued them as standard engineering workstations, and in that capacity, well, they ran text-based software in xterms and Netscape Navigator. That's pretty much it. They will run ancient versions of Blender and similar applications if you really want to exercise the graphics card, but the results aren't going to impress anyone. (Again, about the best you can say is they were "pretty good" for 1998. Sun sold them until 2002, at which point they *well* past their best-by date.)
Maybe I'm the wrong one to ask. Over 20 years ago I *briefly* got excited about finally getting to touch "real" workstations, and ended up inheriting a few Suns and SGIs back when you could get them out of a garbage can for free if you knew where to look, but my enthusiasm for that died pretty fast once I actually had them. Unless you get an extremely high-end model most of these systems are slower than PCs just a couple years newer (in an extreme case, like an SGI with the bestest best graphics they offered, they might still be impressive compared to a PC, I dunno, maybe make it five years newer?), they're loud, they're power hungry, it's almost impossible to find "interesting" software for them (*many* commercial applications for these systems were locked down with dongles or license servers)... again, unless you have something very specific in mind that's going to make a computer that's completely put in the shade by just about any 2002-vintage PC running Linux look good I'd probably search elsewhere for an awesome example of late-90's tech to show off.
Yes, it is more about the aesthetics and preserving it.
How are you "preserving" it if you're putting it in a completely different case? If you just want a weird case connected to a monitor running an X11/CDE you can run CDE on just about any vaguely UNIX-like OS, including Linux, and it'll look pretty indistinguishable from the default 1999-vintage Solaris 2.x desktop.
If you really just want to play with a Sun Workstation I'd probably recommend an Utra 1, 5, 10, or Blade 100, but only if you can get one cheap. The last three on that list are almost as fast as a single-CPU Ultra 60 except for having only 2d rated graphics cards (with the exception of the Creator 3D version of the Ultra 10), and they're at least less beastly. Bonus with the Blade 100 is it uses a USB keyboard and mouse, although you still want to have the *Sun* USB keyboard. (Any old USB mouse will be fine, though.) Then it at least will look like a Sun.