As I write up our agenda for our upcoming trip to Paris (just 3 weeks to go!), I'm struggling with my age-old question - am I over-planning the trip, or am I not doing my homework and am going to end up at all the wrong museums at all the wrong times and wait in unnecessary lines and end up eating lunch at some horrible tourist trap where the crazy lady next to me tells me about one time she had food poisoning on a cruise ship (this happened)?
I've found that my approach changes on whether we've visited the city before. On "first" trips to a major city (Rome, London, Paris), I write out a detailed agenda with each "day" fairly fully planned (and, generally, numbered). To me, this means a list of probably 3 destinations we're hoping to see that day, and maybe a restaurant for lunch, all of which are reasonably close for walking, or a sensible Tube trip. We generally don't do much at night, so that's normally open unless we're seeing a show. If we have day trips scheduled, naturally I include those on the agenda as well. Certain events are inflexible (day trips for which we've bought train tickets, river boat cruises, night openings of museums), but generally, we switch around the days as the trip progresses (hence the numbering - it sounds silly, but it gets confusing when you change Wednesday for Saturday and half of Monday). Some of the most essential things to research in advance (and lock-in on your schedule), are night openings of museums - a must, if they've available, days that places aren't open (generally Sunday or Monday, but not always the case!), or any times that a place is particularly crowded (for example, if something is closed on Monday, don't go on Tuesday).
When we arrive on the first day, our goal is to see as much as we want and stay awake. Sometimes, this means we see a locale (for example, in Rome, we walked by the Pantheon at 4:30 on a Friday and found there wasn't a line - weird - so naturally we went in). Then, from there, we generally adjust the agenda to suit the following days - If it rains on the day we intended to go to Hampton Court, maybe we switch that with the British Museum day. If I'm sleepy one day, we might move days to push back a day trip. I find it works fairly well - we have some form of a plan so that we see the places we want to see in an organized way, but we also don't feel stuck to a rigid plan that would have us standing at the top of Stirling Castle in a blistering windstorm, missing one of the top attractions there - the lovely view! At the end of the trip, we've generally seen all of the things we wanted to see, and in a relatively organized fashion. I don't cram a ton of things into a trip, but what we do, I like to do without an unnecessary 10 mile walk and expensive cab ride back, so this works out well.
If we've visited the place before, for example our most recent trip to London this past spring was my second in a couple years and my husband's third (including a period of living there). We'd already seen the main tourist sights on our most recent trip (there were some I wanted to see again - Westminster Abbey, for example) and some I could pass on (The Tower - lovely and historic, but so expensive without the London Pass). Our agenda for this trip was so much less detailed - some days just had a couple neighborhoods listed. I found this worked well, as we are so comfortable there, that we were able to change up the agenda at speed, and weren't attached to seeing specific things (if we arrived at something we had planned to see and found it crowded, or got side tracked, then we just did whatever we wanted and quickly reconfigured). I wouldn't have felt comfortable doing that in another destination.
Summary: make a plan so you don't end up on a wild goose chase or at a loose end without a schedule (or at the Vatican at the wrong time...see below), but if you see an enoteca you can't resist, ditch the schedule and have fun!