Geocaching is a wonderful idea....
Geocaching.com
I use it to get my kids out and about, and it works! We started out small and close (some a few feet from the car) and went from there. Now they get so busy trying to find a cache they walk a couple-few miles without realizing it. And have a great time. Together you could make some small wood doo-dads to leave as a signature treasure or just little thing-a-ma-jiggies work great too.

I LOVE the wooden toys idea, too.

We also like Project Linus, though that's blankets. They can either be bought or made. We make polar fleece ones, which are pretty easy. You get to shop for patterns you think kids would enjoy.

Feeding the birds. We're constantly calling out to each other to see what's going on at the feeders and stand there watching from the window for a few minutes each day. Buy or make a/some feeder/s. Check with your department of conservation for free plans. Just make sure they're squirrel proof!

Make bird and bat houses together.

Mall-walking/window shopping/no buying allowed. Perhaps a small drink or treat in the food court for people-watching together?

Dinner together at the table, no tv or radio.

Our art museum has a free family art day where you get to see exhibits and create something from the inspiration. We take a picnic which we eat outside, then go in a see what we can make.

Well, what else do the two of you like to do?