Well, tenderloin and standing ribs are always awesome in my book! As well as ribeyes and bone in ribeyes but if they "realllllly" want special...then I would be tempted to do either one of my very favorite beef dishes:
for starters (amuse bouche) I would do maybe smoked salmon blinis with creme fraiche and chives (very small blinis and only a couple per person as I'm serving champagne for the two of them). I would be the server for the dinner and do dinner for 2. A romantic experience.
I would do a roasted caesar salad prepared table side (that means, I would do bagna cauda croutons, fresh parmesan curls, and pre-service I would quickly grill the romaine. Then toss with a freshly made caesar, keeping the romaine hearts in half or quarters and lightly tossed with the vinaigrette. I would also have some roasted tomatoes and parmesan crisps made ahead as a garnish.
beef wellingtons (I do mine with both a mushroom duxelle and a pate de foie gras or else I use a rabbit pate in place of the pate de foie gras that I get at one our specialty stores) then I would be tempted to do a fondant potatoes in duck fat with a bit of sage and served with sherry braised leeks or scallions. I would pair this with a Smith and Hook or Trefethen Cabernet. Or else go full tilt and do a Far Niente Cab.
I would finish with chocolate truffles on a cheese and fruit plate and finish with a gorgeous port or zinfandel or a nice Meritage.
or in place of this menu I would do the same amuse bouche, salad but change to either of these mains:
2" thick bone in veal chop marinated in rubbed garlic clove, salt and pepper. Topped with seared foie gras and tobacco onions and served with a silky pan sauce. I would serve perfectly cubed potatoes sauted in butter with rosemary and sage along with grilled ruby chard with garlic chips and olive oil.
If they can't eat this portion, I would do a veal loin and serve them a cut portion with the foie gras. This is also delicious made with a lamb loin.
Then I would go with a classic lemon french tart or lemon profiteroles served with a lovely champagne. (Since you are finishing with champagne here...I would be inclined to make them a cocktail with the salmon blinis...maybe a gorgeous vodka martini?
But for spring summer, I really tend away from heavy beef dishes like this. Even steak is just soooo much in summer. I prefer a lighter summer menu that maybe uses fish or seafood. Even a pork dish. I love cooking whole fish on the grill stuffed with fennel and garlic and scallions and fresh citrus then serving it with a dynamite sauce, served tableside...