I arrived at my hotel around 730am. That's one of the issues flying from Canberra - there's only two direct flights per day at 6am or 7pm. The hotel had a check-in time of 2pm which left a solid amount of time to sit around before deciding that sitting elsewhere than the hotel lobby may make time go faster.
After a quick search of the Rundle Mall area, I came across high reviews for L&L especially for coffee and toasties. I found it after a short 10min walk (tempted by other coffee places and a food court along the way). It's located in a stylish looking arcade and has a definite hip feel about it. There's sitting space both inside and outside to enjoy relaxing and chatting, using your phone, people watching or pulling out the iPad/MBA.
The coffee looked well constructed, but as a non-drinker I settled for a soy chai latte which was good - not overly sweet, not syrupy, not too heavy in cinnamon, nice. I'd also read about the Bolognese & Bechamel Toastie ($7) which was thickly sliced pressed bread with some nice cheese and meat sauce. I think it could have used more sauce overall as some bites had a bit too much bread and dryness (especially the crust areas) but it wasn't a big deal.
The menu prices are excellent too. A reasonable sized bircher is $5 for a healthier breakfast option.
If I ever find myself straggling during an Adelaide morning, I'd definitely come back here.