Ah the guided tour. Easy without thinking for some, torture for others. I think I fall somewhere down the middle. I have a friend who will be going to Italy for the first time this September. She didn't want to make all the separate arrangements by herself so she signed up for one of those 7 day tours that includes the lodging and some food, and most likely some sight seeing. This is a very easy trip for many people, and I would definitely choose it over, say, cruising Europe. I don't think I would be able to stand being with the same people for a week, and this is why the day tours work so well for me.
Depending on the country and city you will be staying in, day tours can be a good option. Most often they include transportation to a place you would either have to hire a car to see, or maybe its out of the way. It comes back in time to get some dinner, and you're in your regular hotel room to sleep. Viator is a great website I've used to book tours in Ireland, Scotland and Greece. It covers pretty much every country you'd want to visit, and their tours are guaranteed.
Some countries have more centrally located sights where local public transportation is not only cheaper, but easier to get around than using a tour. Some large cities even have buses whose only routes are to sights where you can get off and back on at your leisure. I've used those in Edinburgh, Glasgow and Athens. It saved on taxis, and there is a running commentary telling you about the city and the sights as you go along. They're also very economical, usually about $29 per person.
If you don't plan on doing car hire in Europe, and the public transportation isn't convenient to the site you want to see, day tours might be your best bet. You get a guide who speaks your language and you learn something new. Although if you really want a multi-day tour, Viator has those options too. Here are some they offer:3 Day Edinburgh, Loch Ness and Highlands from London: $565