+1 for Singapore, Cardiff and Glasgow. Having lived in Glasgow, I can definitely say there's more than just knife crime! The food in Singapore is amazing, and so "accessible" for all. I sometimes go there for a weekend from HKG just for food. I'll add a few of my own:

Newcastle - Toon isn't on most peoples' visit list outside the UK, but it's a fantastic city to visit. Lots of famous things to look at, especially bridges and you can soak in the Geordie accents.

Gibraltar - OK... I admit there's not too much to do in Gibraltar because it's so tiny. But with Andalusia next door and Morocco a short boat ride away you can do an incredible variety of things using Gibraltar as your base.

Hangzhou - Usually skipped by most people on their first visit to China, I think it's the most beautiful big city in all of China (and there's some competition!). I love everything about the place, West Lake, historic buildings and of course the tea.

Vladivostok - I was born there so I'm slightly biased, but it's a very unique city because of how far it is from Moscow and how close it is to China and Korea. It's very different than European Russian cities and is pretty much a forgotten part of Northeast Asia, even compared to Chinese Manchurian cities let alone Seoul or Tokyo.

I wouldn't call Calgary underrated at all. Banff, Lake Louise, Columbia Icefield and Stampede are all famous attractions. Rightfully so, because the mountain parks near Calgary are world-class places to visit in any season.