The downtown area around the Delta is mostly expensive restaurants (many of which are good), bland chains, and tourist traps.
In terms of cheap and good, many are mainly takeout. There is a food court in Union Station that includes Roywoods (decent Jamaican), Sansotei (good ramen), and Uncle Tetsu (Japanese cheesecake). Down on Queen’s Quay, Indian Roti House is good for, well, Indian-style roti. Shatter Abbas has good Persian food.
St. Lawrence Market has a number of good places for takeout food. You can read a recent discussion/debate about it here.
Your 45-minute walk radius can take you fairly far. For example, you can get to the Spadina Chinatown, where you can get great noodle soups and fried noodle dishes at Swatow. Mother’s Dumplings has lovely northern Chinese dishes. Yin Ji Chang Fen has great large-size rice noodle rolls.
Just beside this Chinatown is Kensington Market, which has lots of awesome cheap places, mainly takeout. See this report for more details.
In you want one or two splurge dinners, consider Grey Gardens, also in Kensington Market. My recent report on it is here. Closer to downtown, I also suggest Richmond Station for fancied-up comfort foods and Canoe for Canadian-focused cuisine with a spectacular view. My recent report on Canoe is here.
In terms of things to do, that depends on your tastes.
The AGO is a fine art gallery, with a strong Canadian collection. Other art-based museums with an art focus include The Museum of Contemporary Art, the Aga Khan Museum, the Textile Museum, and the Gardiner Museum. North of the city is also the McMichael Collection, which has a great selection from the Group of Seven.
Toronto has some other very good museums, including the Royal Ontario Museum and the Bata Shoe Museum.
Some touristy things that are kinda worth it include the CN Tower, Ripley’s Aquarium, Casa Loma, the Hockey Hall of Fame, and especially Little Canada for all kinds of intricate miniature dioramas.
The best outdoor activity is to take the ferry to the Toronto Islands for a stroll or bike ride. Pack a picnic to go there, although there are a few food options on the Islands.
A nice area to walk around in is the Distillery District. These are all historic buildings that have been converted for restaurants and shops, plus a theatre. If there, at least go to Soma for the amazing chocolate (Old School is my favourite), great gelato, and fine spiced hot chocolate shots.
Hope that gives you some starting points.
