Historic Downtown or "Old Town" Manassas is home to many cute shops and fun restaurants + bars, such as CJ Finz, Philadelphia Tavern, Sinistral Brewing, Public House, Monza, Zandra's Taqueria (features a rooftop), Mariachis Tequileria, Crossroads Tavern (featuring dozens of board games you can play at your table), and Olde Town Sports Pub.
2 Silos is great for a large group. They have their indoor seating and plenty of outdoor space (covered bar-height tables, patio tables, fire pits, etc). They have live music most days and display the schedule for that on their website. Dogs are welcome!
Vertical Rock is an incredibly fun and rewarding experience that allows you to chose guided or solo rock climbing and or bouldering. They have shoe rental as well so you don't have to worry about bringing your own climbing shoes.
I've been to this trail Zulia a dozen times and personally love + recommend it often. The main trail is 2.3 miles and has beautiful views all the way around. It has a paved walking path throughout the entire duration and has varying scenery to include rock formations, large fields, trees, and a pond.
Just outside of Prince William County, this is another great antique store option that George and I have visited (and happens to be 4 minutes from our house). This shop is huge and will have you wandering around for over an hour. You are bound to find something special like we have!
The Promenade is located very close to Manassas and offers a variety of stores and delicious restaurants. Most notably, they have a movie theater if you want to catch a film and Muse Paint Bar if you want to get creative with a glass of wine in hand.