Some good choices there (although personally I wouldn't bother reading Day of the Triffids again).
Le Guin has done several interesting novels on this theme, including one overt utopia - Always Coming Home - which is a beautiful book. Aldous Huxley's Island is also a brave utopian vision. On an ecological utopian theme, I would also recommend B.F. Skinner's Walden Two, and Ernest Callenbach's Ecotopia, which would still be my favourite place to live.
In a feminist vein, if you want to go beyond Atwood, you could go back to Charlotte Perkins Gilman's Herland or the slightly more recent Woman on the Edge of Time, by Marge Piercy, which has dystopia, utopia and reality all in one package.
(this is one of my favourite areas of fiction, as you might be able to tell! I used to teach a course on 'the future of cities' which started with the Tao te Ching and Plato, went through Moore, Milton, Mercier, Owen, and all those plans for future cities and visions of hell - and finished with Callenbach and Greg Egan's Diaspora, which features a kind of utopia of virtual beings. I have a soft sport for Jefferies' anti-urban utopia, After London, from 1885.)