Baths require almost no energy. You even get to lie down while taking them. Your legs don't get tired, they're not in use. You can even get a few hours of kip in the tub if you need it.
Fizz bombs and bubble bath. That stuff doesn't work in the shower, trust me, I've tried. Santa beards and bubble-castles, that kind of thing is for the big boys.
Rubber duckies and other floating toys. I think we all know the importance of company while bathing, and I have complete faith that learning to bathe with others at a young age will increase one's sociability levels later in life. As there's nothing to float on during a shower, one becomes accustomed to being alone and therefore hates mankind when older. You could, in a shower, I suppose, line the soap shelves with on-looking barbie dolls,as my little sister once did, but my father promptly removed them before I experienced their true worth.
Tidal waves, on small, non-life-threatening levels, are wicked, wicked fun. Back and forth, back and forth, get that surf up to a nice height and watch the carnage. Make sure you're on the bottom floor though, leaks only matter if people can see them.
The free hand. It's hard, I know, to resist that temptation of sinking your whole body into the lovely womb of warmth and lovin', but if one does manage to leave out a limb, they have themselves a free hand. Such an asset can combine true passions, for example, the book and the bath, the remote control and the bath, the gameboy and the bath, the yo-yo and the bath, and so on. This is difficult to achieve in showers.
Parties are alright, clubs and discos are ok, but there will never be a better place to boogie than the whirlin' twirlin' shower floor. Get out your extension chord, plug up your means of music, and show the world of soap-suds and shampoo what you really got. Dance like no one is watching. You can do all the slippery moves you only dreamed about, you can work up the biggest sweat of your life, it's an instant-cleansing environment. Take heed, though, I had to have stitches on my chin when I was a wee one; I was doing the hokey-pokey with a little too much umphff and not that much care.
Private concerts. Ever see that comic strip, starring Disney's Goofy, where he's a great singer in the shower but when he gets on stage he sucks, so he has a brainstorm and brings his shower on stage and he's perfect? Well, I have. And I know that whoever wrote it was coming straight from the heart. Newton himself made a long-lost law that singing skills are increased when said singer is in the shower. I don't know why. I don't want to know why. All I want to do is sing my heart out, and the shower is the only place I feel comfortable doing so (and my family allow me to).