Birthday party ideas for 4 year olds and adults?

My youngest son is turning 1 next week and we're having a family birthday party to celebrate.  I'm cool with that.  But then in another couple of weeks, my oldest is turning 4 and I'm struggling with what to do for his birthday.  He'll have a party at school with his friends, but I know our family would like to celebrate with him.  He has 2 cousins close to his age nearby that we could do something kid-friendly with, but I also know the grandparents are planning on coming into town so I'd like to figure out something that we could do that would work for everyone.  I'd kind of rather do something other than another party at our house since we'll have just done that -- maybe go somewhere but I'm stumped on what to do and where to go.  Any ideas? We live in Alexandria just to give an idea of our location.  Having 2 kids with birthdays near the holidays is stressful!!!