"What can you do on a spa day that you can't do in your own bathroom?"
If you're referring to things that leave a lasting visible effect, then I agree with scaryteacher completely!
You may relax more in a spa and get a massage, which is a waste of money for me unless you're happy paying for just the experience (once you're out the door the money has gone and there's no visible or palpable difference to you). The other stuff (facials, nails, scrubs) can all be very easily done at home.
Run a nice bath with some bubble bath, put a hair masque on (the Kingsley elasticiser is unlike anything I've ever tried before!) and a face sheet mask, enjoy reading a book for a while, then use a body scrub (I love soap and glory for this), shave if you wish, rinse hair, get out, massage in a rich moisturiser, remove sheet mask and massage in remaining product, then go and sit in front of the TV and do a mani pedi.
Bottle of radox: £1
Sheet mask: £4 (for a really good one, there are loads of masks under a fiver in boots these days, experiment to find one that really works with your skin, I use the orange soap and glory sheet mask, and once you've used the mask there's enough in the packet for another application)
Hair masque: £15 for a smallish tube of elasticizer
Magazine: £1
Scrub: £10 Soap and Glory do amazing scrubs for a tenner for a huge tub and on three for two usually, also can buy a travel size
Moisturiser: £2-10 depending on what you use
A good base coat, colour and top coat of polish: £15 (I use Barry m which are fab and a fiver a bottle!)
Total: less than £50 for all of the products, all but the sheet mask can be used many times, you could easily do the above ten times over with the above list.
So after the initial purchase you're realistically able to do everything a spa day could do to visibly improve your appearance in your own bathroom for less than a tenner a pop. And you probably have some of the above kicking around already (a book, bubble bath, hair conditioner as a masque).
I've been on plenty of spa days and I love them, they're great fun and luxurious. But there's absolutely nothing they can or have ever done for me that has made me look any better than doing it at home myself. And having the above done at a spa (facial, hair, nails, scrub) would easily cost upwards of £100-200 as every individual thing is considered an extra 'treatment' and soon adds up!
I don't buy that a spa facial does much more for the appearance of your skin than finding a cheap product that suits your skin well and using it daily at home, either. Facials are relaxing but there's only so much they can do in a one off application, and the beautician knows less about your skin than you do, and as she's only seeing you once she can't alter and modify and experiment the way that you can over a period of weeks at home trying different products (which don't have to be expensive).