If you decide you want a more leisurely day trip, there are a few other sights to see in Omiya, and you can do some of your general exploration of department stores and shopping arcades there while you're at it. The excellent Railway Museum was mentioned above, and the
Museum of History and Folklore is also pretty interesting.
If you don't mind a bit of walking you can start at the Bonsai Museum, walk south and drop into a few bonsai nurseries, continue walking south through
Omiya Park, where the museum and a small zoo (with free admission) are located, then stop to admire the beautiful
Hikawa Shrine.
There's a long tree-lined promenade leading from the shrine to central Omiya, and near the station there's a very good cat cafe called
Nekoya - it's less crowded and more comfortable than most Tokyo cat cafes I've been to, although it's crowded on Sundays. There's also a decent brewpub called
Hikawa Brewery (although they're only open from 5:30 on weekdays).