Paris, Amsterdam, and Brussels are all really nice cities. I actually prefer Amsterdam over Paris but all are worth it - but Amsterdam and Paris can both be very busy.
Lille is en route between London and Paris, 1h30 from St Pancras, has some nice architecture and musuems. Much much quieter than Paris!
If you’re happy doing a change, it opens up more places:
Bruges (c.3h25, change in Brussels) - such a pretty city, perfect size for a weekend break.
Ghent (c.3h, change in Brussels) - great food and galleries, fab atmosphere.
Antwerp (c.3h30) - more good food and art! Really good for shopping / window browsing.
Rotterdam (c.3h30 direct, c.4h change in Brussels) - modern art, fantastic central market, great architecture.