HELP! Ideas for mommy/son day

My son came to me with the idea of making a US day (that's how he calls it)

I asked him what he wanted to do but he doesn't know just something we can do every year.

I'm struggling because i want to do something fun that when he gets older he'll still wants to do it but i can't seem to come up with anything.

Do you do mommy/son days?

Should i do various activities & see which one he likes best?

HELP ME PLEASEEEE!!!