Not quite Helen, but some years ago when I discovered that at short notice I would be able to take a week's leave in the middle of a busy term, I went into my local independent TA, gave them the very specific dates I could go away for and asked them what was available in the form of a cheap late deal. What came up was a week's HB in a 4* in Tunisia at a very good price with no single supplements - so I said done and paid the money!
But I did do it thinking that it would have to be really awful not to be good value for money at the price I was paying and that if all I did was spend the week sitting around the hotel reading then at least it was warmer than at home, I wasn't doing the cooking and I certainly wasn't wearing myself to a frazzle at work. As it turned out, it was a great holiday - I booked a couple of trips, discovered that my schoolgirl French re-surfaced enough to be able to get around on my own, that the haggling in the souks was part of the fun and that Tunisia had an amazing history and Roman remains that I previously never knew existed. It was so good that I've re-visited a number of times since.
So I think that the trick to making a success of booking on the spur of the moment to go somewhere that you haven't researched in advance is to be clear about what your bottom line is re minimum and realistic expectations and regard everything else as a bonus, go with an open mind and treat it as an opportunity to experience somewhere new that you wouldn't have otherwise have thought about.
SM