Steve:
Steve takes these rare nights as opportunities to take you out for romantic date. Ever since you had kids there have been fewer opportunities for any particularly romantic dates. So Steve loves to plan a date and take you out for a romantic dinner, both of you dressing up for the night. Your favorite part is getting the chance to see him a suit, since it's a look you enjoy immensely.
Bucky:
You and Bucky like having a night to go hang out with your friends so you usually end up at the bar. You both try to have nice time and relax with your friends, for the most part you're successful, however you do catch Bucky nervously checking his phone every few minutes. He knows your son is safe, but he still worries and wants to make sure he doesn't miss any calls from the other kid's parents in case there's an emergency.
Pietro:
The minute your kid is out the door Pietro is all over you. Honestly Pietro loves having a family with you, but he would lying if he said that he didn't occasionally get frustrated not being able to have you to himself more often. So when he does get these rare nights alone with you he takes full advantage of it and is pulling you off to the bedroom the second you get back from dropping your kid off at their friend's house.
Loki:
The two of you don't have kids, so you can do whatever you want almost all of the time. Meaning you can take time to make whatever night you want special.
Clint:
Unless you're celebrating something special you usually take these nights to just spend time with each other, at home. He cooks a simple meal for the two of you, before you both retire to the couch for a movie. At first it's just cuddling, but before the end of the movie things definitely get more heated and you take full advantage of having the house to yourself.
Thor:
Well, you did want to have a hot tub in your backyard for a reason and you don't have a lot of opportunities to use it. So when the kids are away for the night you like to get cozy in the hot tub with your husband. Sitting back to relax and look up at the stars above you, and maybe even as the night goes on things get steamy from more than just the hot tub.
Tony:
Tony knows that you like to take these rare nights to really relax, so he makes sure to make them extra special. He has one of the chefs that works in the tower make a nice diner for the two of you. Then Tony runs a bath for both of you with candles and music as classic romantic additions, and brings out a bottle of your favorite champagne.
Natasha:
It's Clint's kids that are at a sleepover, and the sleepover is at your house. You and Nat offered to host the kids for a night while Clint and his spouse were celebrating their anniversary. You guys had a fun little game night with the twins and allowed them to have a few too many sugary snacks. It was super fun though.
Wanda:
You two like to take these night to go on more of your adventures around the city. There are plenty of these that you have taken your kids on with you, but there's something particularly special about getting lost in the city by yourselves like you used to. Even if there aren't a lot of unfamiliar places anymore.

YOU ARE READING
Avengers Preferences
RandomPreferences for Steve, Bucky, Pietro, Loki, Clint, Thor, Tony, Natasha and Wanda *Sorry I didn't do them for Bruce. I was having a particularly hard time writing for/about him and I didn't think I could do him justice.