Hi OP.
Congratulations on your pregnancy.
Spa day lover who successfully hid both her pregnancies until after the 20 week scan here.
Firstly, I would call the spa in advance and ask whether any of their treatments are suitable for pregnant women. You might be able to have, for example, a hand massage and manicure, with or without polish. Many women have proper manicures during pregnancy but some pregnant women prefer to avoid harsh chemicals like nail varnish and nail varnish remover; if that's your choice you can just have your nails and cuticles trimmed and buffed. Or they may have a pregnancy massage as an option.
If you're worried about an indiscreet member of staff accidentally revealing your pregnancy in front of the others, I would be tempted to fill out the information card as though you are not pregnant, and then when the others aren't looking you could always nip back and speak to a member of staff and explain that you're pregnant but the rest of the group don't know and you didn't want to risk it being revealed in front of them. You can then swap out a regular massage for a pregnancy massage, or whatever changes you need to make for your treatment to be pregnancy friendly.
Regarding the swimming costume, I wonder whether the one with the ruffles might make a small pregnancy belly pop out even more? I would probably just wear a normal swim suit in a size up from your normal size.
It should be fine to pop in and out of a jacuzzi for a few minutes. The reason they're not considered safe during pregnancy is because they are kept at a constant high temperature. You could have a warm bath at home and that would be considered safe because the water starts to cool immediately, so you only have the first few minutes when it's very warm. I went in my friend's hot tub for a few minutes a couple of times when I was pregnant. You just have to keep it to quick dips and don't let yourself overheat.
Presumably the spa will have a swimming pool which is heated to a normal temperature? If so you can just spend more of your time in there, maybe swimming lengths, which is absolutely fine during pregnancy and since your whole body will be under the water nobody will be looking at your tummy.
I would expect that upon arrival they will provide you with a robe and you can spend the whole time you're not actually in the water wearing that, which means nobody will be looking at your tummy or notice a bump.
Bring a book, magazine or Kindle and just find yourself a lounger and settle down to read.
Basically, just do what you can safely do and don't make a big deal of the fact that you're not doing the other things, and I bet nobody will notice. If anyone tries to get you to go in the sauna or steam room, just say you don't feel like it, or don't really like feeling overheated.
For the wedding, any dress which isn't too tight round the tummy should do the trick. If you are slim with good legs then you could go for something with a fairly boxy fit which is loose around your torso and shows off your legs. If you are more curvy then I would go for a maxi dress which is fitted round the boobs and then skims gently over your tummy all the way to the floor, with flat sandals for maximum comfort. You could wear a statement necklace or pair of earrings which will draw people's attention to your face and neck, meaning they are far less likely to be looking anywhere near your tummy.
You've got this!