Hi Duckduck, I am very careful to feed all my horses pregnant or not, top quality supplements. I fed my mare Merlot a quality brood mare calcium mix on top of her supplements from day one. It depends on where you are from as to what the vit/min mix should be as some soils are deficient in things like selenium and some have too much - you would have to check with your vet on this.
If you are in Australia or NZ I can not speak highly enough of the ProvideIt range of vit/mins available from their website ProvideIt Vitamin Mineral mixes for horses.
I attribute my boy Zephyr's bouncing good health and energy to this range.
Also feed plain SALT in their feeds whenever they are on green grass - 3 tbsp for an average sized horse. This is vital and to be fed on top of a salt lick in the paddock. (this is to counteract the high potassium levels fund in green grass)
As for exercise there are a million opinions on this one - just use common sense - once the foal is in the third trimester the weight of the foal increases hugely by the week - I don't believe in adding to this
Apart from that I believe in forming a strong bond with the mare during the months left to foal so that she doesn't mind you being around when she foals. This includes visits to the paddock where you just BE with her.
For me this resulted in Merlot waiting for me to be there before foaling at the very convenient hour of 4.30 in the afternoon (rather than in secrecy in the middle of the night)
That done, sit back and enjoy the wait! Good luck