Oh, goodness, what a good son you are. Your parents should be proud.
I think the Rolex idea is very generous of you, but if your dad wouldn't wear it at all times, then it would be a huge amount of money spent for something that would stay mostly in a box.
And yes, you are correct, it would be better if the gifts had similar value. You don't want to inadvertently send the wrong message.
As a parent, the most precious thing to me is time. That is priceless. A gift I loved from my son was a hand made coupon booklet. The coupons ranged from one hug on demand to making breakfast, to going out to dinner.
I was thinking you could take each of them on a weekend somewhere they would like, somewhere they would never go on their own, or always wanted to go but never got around to it. Take pictures, frame the pictures, so they can always have the memories to see.
Does Swarovski make a locket? If she wears jewelry, might your mum like a locket with the photo inside?