You need a big squishy sofa that can double as a bed if a kid (or you needs to crash out) and loveseat. Get washable slipcovers for both.
A low and plain coffee table that’s easy to spray & wipe clean, kudus to the ones that have drawers built in for storing DVDs and game controllers/remotes.
A corner desk with room enough for a PC and laptop to be worked on.
A great big TV (my fiancee’s suggestion)
A complete stayover kit under a spare bathroom sink for when the kids come. Have basics like toothbrushes, toothpaste, deodorant, sunscreen, babywipes, bandages, shampoo, conditioner, scrubby sponges, pump soap.
A collection of board cames or decks of cards, a wall mounted dart board, laptop side table.
Buy yourself two sets of nice sheets, two nice quality blankets and a comforter or duvet cover. Same with towels, get the big bath sheets and at least 4 of them. Keep babywipes, lotion, toothbrush, toothpaste and some nice bar soaps under your sink for guests.
IKEA is one of the least expensive places to shop for household goods that are bachelor and bachelorette friendly. Check out the website, create an account and start adding things you like and then go back and edit until you form an idea in your mind of what your new digs should look like.