If you would like to eat just a snack or coffee and have a great view, the one of the best places is The Ginger Gallery & Cafe. Its number 86 Hotwells Road which is the A4 road leading out of the city to the West. The main attraction about this place, besides the art / craft / jewellery which you can buy, is that it has an uninterrupted view directly opposite to the SS Great Britian and the main dock area which is always good to see - lots of activity and people watching. Its not a very big cafe (upstairs) but its certainly worth the 10 minute walk from the city centre.
If you are on your way into the city via the docks area, possibly just after visiting the SS Great Britain ( www.ssgreatbritain.org) there is a cafe situated on that same quayside towards the city centre which is called 'Brunels Buttery' and is reputed to do the best bacon buttie in town!
A few more places to eat are Cafe Unlimited, St. Werburgh's Farm Cafe, Harveys Restaurant, The Glass Boat Restaurant, and Oliveto.