Prepare for a snowball fight!
Well, you rarely plan a snowball fight; in fact, it's more often a snowman build that gets out of hand. My little piece of advice: start with the fight to avoid tears if the snowman suffers collateral damage from the battle.
So to have a snowball fight, you need snow and a pair of gloves. Yes, you were right to come and read this tip. Sorry, I'll keep going. So, as well as snow and gloves, you'll need to bring a scarf or hood, because snow sliding down your neck is unpleasant (sorry to bring back this good memory). A little tip for encouraging kids to wrap up warm: tell them that their ski suit and jacket will cushion the impact of the balls coming their way.
Then you need some land. You know what I mean: big enough, with snow that's not too hard, no gravel or pebbles, and a few hiding places. And if you have an abundance of snow, you can even improvise building defensive snow walls.
Finally, a few rules are in order. Saying that, the only one you'll manage to get heard at first is "NOT THE HEEEEEEEEEEEAAAD!". Then you can suggest others. But first...