I am definitely an overplanner. Well, there are three "levels" of planning - the long term (which trips to take which year), the mid-level (exactly which dates a trip will encompass), and the finer detail (what goes on each day of the trip). I've even been known to take that last one down to 10 minute intervals.
For the long term, we know that our family basically gets one vacation a year. And, we know our kids' ages each year and how long we have them until the oldest heads off for college. A few years ago, we made a list of the main trips we wanted to take, and then plotted them out over time, trying to figure how old kids would be and how they would enjoy the trip at that age, whether this was a special event year or something (e.g. our 25th anniversary), etc. We weren't real specific - we have things like: "Grand Canyon and Southwest US", "Washington, DC and Virginia", "New York and New England", "Colorado, Utah, Yellowstone", "Orlando", etc. It's kind of a bucket list thing. Over time we've modified it a bit - we moved our Southern California trip earlier than planned when I was able to combine it with a conference, for instance. This might seem simple, but a spreadsheet really helps keep the various considerations in mind.
In mid-term (our real trips are in the summer time), we make out a calendar of all the days of summer and start blocking off the known activities and so forth. Usually we can't do this very well until early Spring, and we have had to sometimes take a best guess for more advance planning. But, usually we work out the details once we can look at a day-by day schedule of the summer, and make sure we are leaving sufficient times between activities (e.g. not starting a trip the day after a kid gets out of camp), etc. Usually, even if we had to make some really advanced reservations, there is some flexibility (in terms of several days) in how we can structure things. For this, I create a large table (I actually use powerpoint, but almost any Office application can work) and put hard constraints in, then we print it out and start penciling in stuff. It's better than a wall calendar.
For the fine detail, I keep a Word file. A format that works well for me is to have one page with an overview (just what is happening each day, and some key info like when we have to leave on day 1 and get back at the end. Then, each day gets its own page (or pages), where I can list all the details: reservation times and confirmation numbers, options for meals/events/trips/etc. It's basically chronological. At times, I'll get really detailed and use a spreadsheet to track time all day long. And, while it's true I will sometimes go down to the 5-10 minute level, it's not that we try to stick to that schedule - it's just so that I can see whether the things I have planned will actually fit in the given time. I can't tell you how many times it seems like some "quick" task (I mean, even just take a bathroom break for instance) actually can throw off the plans. It actually helps me make sure I'm not trying to do too much (my tendancy) in a given time, and then getting disappointed when reality does not conform to my schedule. When we actually travel, this is what gets printed out, and we often look over the next day's plans each night, and sometimes take the printout with us during the day. On one trip (this worked really well) we had a small binder with a pocketed divider for each day. We would put things we wanted to keep (brochures, maps, etc.) in the pockets to correspond to each day. It made it really easy during the trip.