My family just returned from 3 weeks in Europe (England, Sweden, Germany, Austria, Luxembourg, France). With such quick trips, we wanted to pack lightly (eBags TLS Mother Lode Weekender Convertible) and did so with great success. My biggest concern going into the trip was our flight weight restrictions between Sweden and Munich on Baltic Air which allowed only a combined weight of 17.6 pounds for a carry-on bag and personal bag. We were all just under that weight. I appreciate all the help I got from this site and hope I can help others too with what I learned.
Laundry
Laundering in a sink worked just fine with bar soap or even shampoo/conditioner. I packed an 8-foot piece of paracord and some miniature clothespins. Wringing out the clothes well to where they didn't drip was all it took to dry them overnight. We only had to wash our one pair (each) of jeans once or twice during the trip and they were fine. We wore shorts most days anyhow, but it got chilly in Sweden. We normally hung the line near the open windows where we were staying.
Clothes Selection
The 4 of us had 2-3 pair of shorts, 4 or 5 shirts, 2 long pants (jeans/capris/skirts) and undergarments. We borrowed jackets from our Swedish family we were visiting when it got chilly at night. Towards the end of the trip, the kids bought another t-shirt or two.
Travel Ease
As we traveled on multiple trains and planes, I found my wife and daughter repeatedly mentioning that they were happy we packed lightly while watching others struggle with rolling suitcases down cobblestone streets and airports. Not having to check bags or collect them once was so nice too.
Security
I used the Pacsafe Pouchsafe PX15 for my day-bag. It was super light and the straps were pretty comfortable. I did get sweaty on my lower back on long walks where it contacted the body. I guess that is unavoidable. It did have one interior pouch that allowed me to feel comfortable walking around towns without worry that someone could open up my bag on a tightly-packed subway or bus. I also used a travel lock on the exterior zippers as over-kill in certain places. For the record, I never felt all that threatened on the trip, but we looked out for each other and kept our bags in front of us.