If looking at Hongdae, some of the good options (my opinion)
[Holiday Inn Express Hongdae] - the rooms are quite small, but very clean. Comes with free breakfast which is really simple, but decent and nothing to complain about. Location is good. Near the stop for the airport express and located at the intersection of Hongdae and yeonnam (one of the current hotspots of Seoul). But the pricing is quite strange at times, I will say if it’s less than a 100k krw, it’s a good price.
[L7 Hongdae] - the lounge at 21st level is cool. Nice place to relax and enjoy the view. The bar at the top is also good, though the drinks price are a little expensive for a 4 star hotel. The swimming pool is really famous, but closed for winter. Room wise is larger and nicer than holiday inn express, but like above, can get quite noisy on lower levels. Love their malin goetz toiletries too. Do note that L7 does not have room service. Location wise, right beside Hongdae station, about 5-10 mins walk to yeonnam. About 10mins walk from Hongdae airport line.
[Ryse] Much more expensive than the rest. But much fancier lol. Breakfast is ala carte though. Very design centric and gym is quite nice. Rooftop bar is good but a little expensive. Has a bakery Tartine on the first level which is one of my favourite bakery in Seoul. Room service available. Abit further than L7 from Hongdae station, but not by much (1-2mins?). But that also means it’s closer to the neighbourhood of Sangsu (10mins walk) which is another new hotspot in Seoul with all kinds of cafes and restaurant - if that is your thing.
[Mercure Hongdae] - Very new hotel and quite design centric. Opposite L7, so in terms of location it’s the same. Rooms quite standard though they have the amazing junior suite rooms which has an open air bathtub, quite cool. But I think that room is pretty exp - around 300k krw. Overall, unless u book junior suite room, or have Accor membership for breakfast, I think I will prefer L7, considering their price is around the same.