Is it easy to spend time in Bangkok without speaking Thai?

I would say its easy to get by speaking only English, as another poster stated most taxi drivers can't speak english, or maybe a little. However, the ones I ran into who couldn't understand seemed to have someone to call who knew English and could translate. If you do know a couple key phrases such as Hello, and Thank you, it goes a long, long way. If you only learn one word in Thai, learn how to say thank you, I guarantee the respect you will get from Thais will multiply, and will much appreciated. As for being a woman in her late 20s, I don't think it will be a problem, but I wouldn't flirt with Thai men, that you have no intention on getting to know better, it may be construed wrong.