Hey Earthwalker. Great question. I too am unwilling to dance in a backpack.
I have a day bag and a night bag, packed before I hit the playa. (A list of stuff I want to have in both is sharpied on the inside where it won't bleed through. I restock daily.)
The night bag is much smaller, and pared down to my essentials. I don't like to take my camera out with me at night, so out it goes. I'll risk eliminating dishes (bowls or plates) but I'll keep cutlery, a canteen, a collapsible cup, a cocktail, some snacks, minimal first aid, goggles, eye drops. I often don't ride a bike at night, so I don't carry bike stuff at night. No hat at night, just a shemagh for dust. And I'm willing to wager that I won't be out late enough to require sunscreen the next morning . . . although someone with very fair skin and a penchant for being up until 8:00am might make another
decision. And I'm probably going to check the status of my batteries before I head out, rather than carry extras. Again, that's just me. I might have small spares of things on my person. If a need is greater than what I have, they can come with me back to camp, or I can help them find something better.
So that's the kind of thought one can can put into it . . . customized for your own preferences.
As for the fate of your bag: take it off and either lean it against something but still visible, or gamble and hide it under something. Be emotionally ready for it to disappear, even though bag theft isn't rampant. Tools, bottles of booze, and bikes disappear most. Your bike absolutely WILL go walking off without you if you leave it unlocked. Light it distinctively to pick it out in a crowd, and lock it at all times, even to just itself so it can't be ridden immediately.
You might also find that your neighborhood has a good bar or a good dance floor only a few blocks away, and do that sometimes with very little equipment on you. I love dancing "locally" (and in the afternoon sunshine). It's a warmer experience, figuratively speaking.