I am having a party in two weeks..sort of family reunion so all ages. It will be at 7:30 so more meal food but buffet/finger food style. You could easily do a dessert bar with maybe some fruit and cheese trays too.

Brisket
Ham & Swiss cheese sliders
Smoked salmon
Chinese chicken salad in won ton cups
Mini Twiced baked new potato
Mushrooms stuffed with brie
Veggie tray with tomato caprese, roasted red pepper hummus dip, spinach/artichoke dip, veggies
Queso & chips

Dessert Table
Red Velvet cake balls
Lemon Raspberry cake balls
Key lime cheesecake bars
Mini deep dish fruit tarts
Pecan pie bars

I'm not sure what you meant by not having to serve a sit-down meal, but I hope it doesn't mean that you think you don't have to serve food. That is definitely lunch time.
If I were invited to an open house 1-4, I'd assume that meant you were serving lunch. Not a seated meal, but substantial enough food to make a meal, not just tiny nibbles or finger food.

If you want to or can only serve less food, start no earlier than 3 pm and go no later than 6. You could also do 8:30 pm - midnight, but I doubt you want that for Grandpa's 94th birthday.

If you want to stick to 1-4, you could have a sandwich or pasta buffet or a salad buffet without it costing too much.

Thanks for the help! We had the party last week. It was a surprise and his grandpa was very appreciative I knew we needed lots of food. I just didn't want to serve something that required you to sit at a table. We went with a menu similar to what texanniew suggested with a Polish twist - ham, sausage, meatballs, pierogie, and lots of salads. We had several tables inside and out for people to sit, but everything was bite sized so people could walk around with their plate too. It worked out great. Only problem we had was with the dessert table. I put the birthday cake in the center with the candles on it so it was obviously THE birthday cake. Someone cut a slice before we got to singing Happy Birthday! Lol. Who does that? No one fessed up and the birthday boy got a chuckle out of it!