Found it! The book is called Microadventures: Local Discoveries for Great Escapes
by Alastair Humphreys. Really fun, doable outdoor ideas. A few of them are:

• Just sleep outside, in a sleeping bag, in your yard
• Climb up the nearest large hill. Doesn't have to be a mountain
• Go catch a fish. Cook and eat it. If you don't catch it, go hungry

It's a fun read, inspiring.