Mermaid, there is a savory cheesecake with sundried tomatoes in it that is wonderful. You can make it well in advance of the party and serve with a selection of crackers or breads. We prefer thin slices of dark country breads (like olive or walnut) over crackers these days. Cheese straws are also a favorite of everyone. I like to make the dough and then cut out star-shaped shapes, place a pecan half in the middle of each. We stack them into a little pyramid, something like a cookie tree at Christmas.
Costco does have some good selections, including the shrimp platters and baked bite-size hors d'ouvres. While they have become a huge cliche, those little bite-sized quiche hors d'ouvres disappear instantly at our parties or we can use the leftovers for breakfast the next day.
I agree with BeachRunner about the catering of parties. Unless you really enjoy the cooking and have the energy for both party and partying, the caterer is the way go to. We hire a personal chef from one of the local college programs or wineries, who charges a very reasonable rate and brings a pal along if he needs prep or serving assistance.
Also, I have stopped keeping cookbooks at the beach house. One can find a dozen or more recipes for virtually any dish or category of food imagineable on the Internet. I just boot up the laptop and do a search keyed from "Recipes." I would say the best I've found is the one called "Epicurean, etc." These recipes also include ratings and comments from those who have tried them.
Good luck with your party and Congrats! on your daughter's graduation, Mermaid. :clap_1: