For downtown lunch, Ganga (200 block 8th Ave SW) is halal Pakistani/Indian, and not too bad, with loads of desserts. (I took Arabic once a couple of years ago, and "halal" is the only word I recognise.)
I'd assume Sahara is halal as well; haven't been there in a while, but my coworkers were there recently (shortly before Ramadan) and really enjoyed it. They do a Lebanese lunch buffet, which is a great way to do mezze for only one or two people.
Sahara has a lunch buffet M-F which is delicious and is 100% halal. I actually prefer the buffet to the a la carte menu there- something about being able to dive into a platter of hummous and matlouba is like a dream for me.
As makan notes, there are Indian buffets that are halal. I don't know about the buffets but would take makan's suggestions as good ones: additionally it's not a buffet but Tiffin's is Ismaili-owned and is as such halal too. They serve thalis in-house that are very good value with lots of food in them.
Try this Indian lunch buffet - very inexpensive only $9.99/person - home-cooked food
Dixi's Choices Cafe Bistro
705 5 Street SW
Calgary, AB T2P 1V8
I think Raj Palace might also be halal.
731 - 6th Avenue SW
Calgary, AB T2P 0T9
Another restaurant is Taj Mahal:
4816 MacLeod Trail SW
Calgary, AB T2G 0A8