The Harbourfront/Union station area doesn't have a lot of good food options, so Kokoro is a pleasant surprise. It's a takeout place, although there are a few tables set up just outside. I would suggest just getting your order to go and finding a nice spot by the lake for a picnic!
The Korean owners are very cheerful. They'll make your order pretty quickly, but if you want to be super-efficient, you can phone in your order in advance. They have some great sushi roll combos, ~18 pieces for $11-14 depending on the combo. Rolls are fresh and have a good fish to rice ratio (not overloaded with rice like at some other budget sushi joints). You can sip free green tea while you wait.
They also sell desserts like mochi and red bean ice cream.