But sometimes you are faced with an opportunity to do something, and for whatever reason you choose not to. I don't mean that it comes down to time or cost: the opportunity is possible on both counts. But you just don't do it.
We've experienced this a couple of times, and those experiences have really made us aware of the need to take the opportunities that are presented. On our first trip together, Yvette and I spent a week in Vienna. We had the opportunity to take a day trip to Prague. We decided against it, and instead decided to go out to the Danube River and chill out. It was a decision we have forever regretted. The Danube, as it flows through Vienna, is not picturesque. Upstream, yes, but through the city - no. It is neither beautiful nor blue. We could have been in Prague, but we weren't.
In Scotland, we drove across the highlands from Stirling to Fort William. On our way we stopped for lunch at Blair Atholl - practically outside the gates to Blair Castle, which we decided not to visit. It was only years later that Yvette realised that we should have gone in, that the castle was the home of a magnificent collection of textiles that she would have loved to see. Obviously we didn't know this at the time, but we had the opportunity and failed to act.
In planning our trip last year we originally planned to do a side trip to Tangier, Morocco. We regret not doing that on one level, but on another the idea of taking two small, fair-haired children into a bustling, manic, culturally disorienting North African city just didn't feel right. We don't speak any version of Arabic, and felt that it would have been enough of a struggle to get about as adults without having to contend with the kids as well. We're still planning to include Morocco one day - but we're saving it for when the kids are older.
I guess we're fortunate that our real regrets have been so few, but the advice still stands: seize every opportunity you can while keeping in mind that you can't do everything.