rehoboth beach de
Haven't been in several years, but in the past have had some very decent meals at the Atlantic Seafood Grille at the Atlantic Sands Hotel which is right on the Boardwalk in Rehobeth. I've never been to Victoria's Restaurant at the Boardwalk Plaza Hotel, also on the boardwalk in Rehobeth, but it always seems crowded during the summer. Both hotels are open all year.
lludra, When is your dinner? Do you want to stay in that Rehoboth area or would you consider the other nearby beaches? Also, do you literally mean you want a restaurant "on the beach?" So many of our restaurants are closed for the season.
Lupo Di Mare is a new Italian Restaurant and is on Rehoboth Avenue. It is beautiful and is open all year. I have not eaten there yet but it is owned by the same group that runs Fish On in Lewes which never disappoints. For steaks, you might like 1776 but it is on the highway. Victoria's is on the Boardwalk and would probably be a good choice because its menu is so varied. The Buttery would make a 70th birthday very memorable. It is on Second Street in Lewes.
You could have a wonderful dinner party at The Backporch on Rehoboth Avenue but it is closed until May. My son loves Just in Thyme which in on the highway toward Dewey. Nage is definitely a special occasion restaurant but is also on the highway. You might like the Parkway in Bethany for such a special occasion, or Baywood in Millsboro if you would consider outside Rehoboth.