The idea of a week's tour on a narrow boat intrigues me. The travellers will consist of my husband, my elderly (but quite mobile) parents and me. My parents are used to travelling in a larger RV, so I don't think they'd have any problems adapting to a narrow boat as far as amentities, etc. My husband has problems with his back and as a result doesn't do much heavy lifting, etc.

How strenuous an activity is manipulating the locks on the canal? Likewise, does it take much effort / knowledge to pilot these boats? Are these generally round-trip itineraries?

Our timeframe for this trip would be either in the Spring (late April / early May) or Autumn (mid-September / mid October). Aside from the fact that weather is always unpredictable, which would be the better time to take this trip?

So far, I've looked mostly at the Llangollen Canal. Is this a good choice for a first-timer or would we be better off with the Monmouthshire-Brecon Canal? We'll be spending a total of two weeks in Wales, so whichever one we do, we'll catch the other area by car.

We did a week on the same canal and loved it. The locks are not too strenuous and we had no previous experience but we picked it up quite quickly. The canal passes over a huge acquaduct which is a treat. We rented from http://www.maestermyn.co.uk/ as did Harrison Ford and Calista Flockhart when they did this same stretch of canal!!!
They were very helpful and have a parking area which is secure for the duration of your trip.
If you need any more info just ask. We have also done the Oxford canals too which we enjoyed as well

I've never been narrow boating, but it's always seriously intrigued me, so I've kept this bookmarked. It may interest you--it's a noncommercial site with loads of links, FAQs, trip reports, etc.

A friend of mine was on a canal holiday this summer and he told me he was intrigued one early evening as he watched a boat make its way slowly down the river stopping at each boat it came to. Sometimes a package would be passed over. The boat finally came to him and it turned out to be the ice-cream boat - just like a land ice-cream van. It sold ice-cream, sweets, cigarettes and assorted other things you might have forgotten.

I had never heard of this before but it shows the nation of shopkeepers is still there somewhere.

Either of the canals you mention would be an excellent choice. The LLangollen has enough locks to be interesting (a good chance to chat to other boaters) but not too many to become tiresome. The Mon&Brec has (I think) no locks. The LLangollen is the most popular canal in the country, and I would not advise going there in high season (July/August). The Mon&Brec is isolated from the main network of canals, and is much quieter. Both are very beautiful indeed. If you want a VERY quiet trip, you can hire electric boats on the Mon&Brec.

Locks are not difficult to work if you are fit enough to walk (each lock requires a certain amount of walking back and forth from one end to the other). On a canal with many locks, you would get quute tired, but neither of these canals are likely to exhaust you. It will all be explained at the hire base, where they will show you how to drive the boat (one lever for backwards/forwards, and a tiller - like a sailboat - to steer with). Thousands of people take their first narrowboat trip each year - you won't be the only beginners, and canal folk are very friendly and will help you out whenever you need it.

If, on consideration, you are daunted by the thought of driving a boat and working the locks yourself, google "hotel narrowboat england" and look at a trip where someone else does all the hard work and you just sit back and relax !

Almost all narrowboat trips are round trip - you have to get the boat back to the base you hired from. So you either have to do a "ring", or and "out-and-back". The problem with rings is that you commit yourself to too great a distance and spend the second part of the trip racing along to get back in time, with no time to stop at pubs or explore villages, or have long leisurely lunches on the sunny back deck. With an out-and-back trip, you just turn round halfway through the week or fortnight.

Tips
1) You steer a narrowboat standing up at the back exposed to the elements, so be prepared for a fair bit of standing and a fair bit of rain !
2) Don't overplan. If your trip would be spoiled by not getting to everywhere on the map, go by car instead. Narrowboats travel very slowly (that's the point - it is a relaxing holiday, not a white knuckle ride). Be prepared to turn back without getting to your planned destination when you are half way through your week. Just enjoy the pubs, the villages and the scenery you glide through, without being too fixated on getting to a particular place. You will average LESS than three miles (or three locks) an hour - so three miles and three locks will take more than two hours. Count on averaging no more than twenty locks plus miles per day. Expect to do more, and you will be anxious and frustrated and looking at your watch all the time.
3) Don't overpack. Narrowboats don't have a lot of storage. You won't need many clothes - very few canalside places are posh. Dress for comfort, not for looks. And you can wash clothes on the boat and dry them in the fresh air ! Don't take hard suitcases, there will be nowhere to put them and you will be falling over them, and moving them around, all week. Take soft bags than can be squashed into a drawer.
4) Don't skimp on your boat - the boat IS your vacation. Make sure there are enough fixed beds for all of you. Turning the dining table into a bed at night, and turning it back in the morning soon becomes VERY annoying. An look at the arrangement of bedrooms and bathrooms to make sure you all have sufficient privacy (especially if someone needs to go to the bathroom, or to the kitchen, in the night). But otherwise don't get a boat that is too big. The bigger the boat, the longer it will take you to learn how to thread it neatly through brigdes and into locks. On these narrow canals, you only have about six inches on either side of the boat as you go through a bridge (but everyone makes loud banging noises in the first couple of days, and the boats are made of steel !).

Do read some trip reports on the Web. Google "Narrowboat trip England" (or Wales) and see what other Americans thought of their trip.

If you like green countryside, and English (and Welsh) pubs, and like meeting people, and are willing to try something new, and aren't in a hurry, and don't require guaranteed sunshine - go for it. But beware, you will not want to hand your boat back after a week.....

This is all helping me quite a lot, thanks!

Next question (thanks to my father): Are we going to run into any problem with insects (mosquito-y types that will annoy us)? I know that many people talk about midge season in Scotland. Given that we are considering either May or September/October, is one of those choices better to miss bugs?

We have a few pages on SlowTrav about Canal boats that don't answer your specific questions, but might be fun to read.

England Canal Travels - The Avon Ring, by Lucille who does travel planning for England canal trips

Canal Boats in England, by Jack who took his family on a canal boat vacation

Walking the Kennet and Avon Canal, by me who likes to walk on the paths along canals