Happy 75th to your Dad! How about having some "oldies" going in the background and have tables arranged so there can be dancing going on after dinner? Or sending guests a form (well before the party, with instructions to get that form back to you), and have each person write down a good memory incorporating your Dad. He will cherish these. You can put the filled out forms into book form (or put nicely into a scrapbook) and he can look back on this and enjoy it for many years.
It might be fun to dig up some info from the year he was born and stand up, maybe in between courses and announce what the price of milk was in the year Dad was born, who was president, the most popular music, dance, etc .
Some trivia games (with small prizes awarded to winners, sometimes), possibly historical questions (again, around your dad's year of birth) or seeing who can answer the most questions about your Dad (questions can be as simple as his favorite food, color , favorite vacation, that type of thing).
Celebrating such a stone is always a lot of fun. It's a time to reflect on the years past and share all the memories the family and friends have of the guest of honor.
Family trivia questions with dividing up the guests into teams can be lots of fun, and if you are someone you know are technically savvy you can put together slide show / video of his life from the begging until now … choose songs that fit the era the photos are from.
Get the entire family involved by making some very special memories for your dad. Ask each child and / or grandchild to write a one-page memoir of his or her most special memories. Then put them into a scrapbook and display it on a table for all to see. Guaranteed he will still be looking at it years to come.
You might want to have someone with a digital or Polaroid camera take photos of the guest in their black and white garb … in a simple white frame they have a nice little souvenir of the party.
To follow the theme you might do a short trivia game, teaming up guests, 3 to 4 to a team. Make all the questions about your wife ranging from very easy to impossible to answer. Guests get into a very competitive mood and have a lot of fun trying to answer the most questions correctly.
At the "sound of the bell" or time's up whistle teams pass their answer sheet to the team to the right (does this sound a lot like school ??) and the answers are corrected. Prizes for the team that gets the most correct. Trivia questions should be from her childhood (family and oldest friends can answer those) her school days, her work days, and her current life. About 20 questions are good.
Those are both activities that are fun and memorable. Then with the visiting, music, food and drink you will have a beautiful beach party.