Doing something you enjoy is very important for any "additional" income. As you mentioned, when you are already tired from a day's work, additional work can seem like a heavy load. Skill comes with doing, so if you target something you really want to do, then the rest will often come with time.
If you are comfortable designing/building websites, you and your wife could start a site or two about topics of special interest. The income can come from advertising. That can be a lot simpler than trying to sell products or service online (although becoming a merchant is not so difficult if you enjoy selling).
Google and similar ad network services (like Commission Junction, etc.) make it pretty easy to put ads on blogs and other websites. The ads with these services are usually keyed to topics on the pages. The system is by no means perfect, but it can allow you to focus attention on your own interests instead of spending time selling ad space, billing/collecting for ads, and so on.
I started a couple of very simple sites a few years ago and now have a decent income from ads on those and other sites. Not enough to live on but enough to make my time and effort worth while. I don't sell anything. I just do what I enjoy doing. The ads do the rest.
I prefer any kind of truly independent business income over joining any of the sales and marketing programs out there. That's just how I'm wired, I guess. I actually enjoy sales and marketing, but find it difficult to get excited about anything built on hype and manufactured enthusiasm. Many great products and services are ruined, in my opinion, by the silly schemes developed to sell them.
Jim