Japanese is always a hit with thais, Youngone has recommended two good restaurants in this thread here (Zuma and Tsu Nami) :
But you can go cheaper also, many of the malls are swarming with Japanese restaurants, places like Central Embassy and Emporium have plenty of mid to high end.
Following that, you could head to Gold Class Cinema at Embassy, I often see guys take dates there (even LB's).
Or if you prefer to drink, Maggie Choos in Silom (below Novotel) is a LB friendly speakeasy style bar and is themed as a British East India Company bank vault, they frequently have live jazz and blues/soul bands/singers.
Or for something more elevated, cocktails at Moon Bar in Banyan Tree Hotel, probably the best night view of the cityscape, she will love taking selfies up there. Pretty busy spot usually though.
If you'd rather spend less, but show you know some trendy hidden gems, try Tep Bar in Chinatown: https://maps.app.goo.gl/jUnCq
If an activity is preferred, then get on a river cruise, book online, some have bars.
Or head to a night Market, for a date the flower market used to be best but that's a shadow of its former self now, so another good market is up near Bang Na, its an antiques/memorabilia market called Rod Fai Night Market https://maps.app.goo.gl/KhfF3 this place has loads of interesting things to browse, maybe buy.
Alternatively, for somewhere close, Neon Markets (Talad Neon) on Petchaburi Rd is good for selfies (because thais love selfies).
Chock dee!