I was there a couple of weeks back.
There are 3 price levels. I went for the basic one which is use of changing rooms/lockers, towels, a free face mask and a free drink.
The two fancier packages included slippers/dressing gowns and possibly food and dedicated private changing room.
You have to shower before entering the pool, before leaving the cubicle slather the provided conditioner all over your hair, don't rinse,, enter the lagoon as is. Condition well again when you shower and leave. I did that and hair was fine. There is a sauna and steam rooms.
I did it after arrival at the airport - you can book Flybus, they'll pick you up at the airport, and drop you to the lagoon. There is a building just for storing suitcases for a few krona. Some people do it as they head back home but then you're more pushed for time. You can then take Flybus from the lagoon to your hotel. Works just fine.
There is an expensive cafe on site, think £16 for a panini.
Golden Circle is very nice. It was dry and warm for the time of year when I was there. Two days later travelling through the same area coming back from southern Iceland it was all covered with snow so it's a matter of luck with the weather.
Make sure you take warm coats and good footwear with you.
Did snowmobiling at Gulfoss (one of the GC stops), recommend that. If you've spare time, go to south and see the glacier lagoon and diamond beach and some beautiful waterfalls but it depends on how much time you have.
Don't worry if you don't see NL. I was there for 8 days and didn't due too cloudiness.