Paint is your most inexpensive solution. Painting metal siding requires the right temperature and primer for it to last a while. Since this sits in the trees, allow me to suggest that you paint all the metal siding a dark park green or cottage red, and most of the larger wood members can stay natural - If you are keeping them, paint the little sheds that have been put under the lean to in the same green/red - use some dark white / light tan on the all the trim. I'm thinking of colors like:

http://www.benjaminmoore.com/en-us/paint-color/rainforestfoliage

or bm pottery red http://www.benjaminmoore.com/en-us/paint-color/potteryred

you might paint all the squares in the grid the same dark tan http://www.benjaminmoore.com/en-us/paint-color/northamptonputty

or do the grid in green and white

If you decide to paint everything but the big beams- paint a dark white trim everywhere

http://www.benjaminmoore.com/en-us/paint-color/limewhite

and use the putty tone on wood that just needs protection (inside of doors, etc.)

Just masking and painting (and organizing the clutter that inevitably occurs over time) will make this nice. Consider adding some shade sails in a khaki tone - between the building and the trees - that will give you more to work with and help the lean to architecture become even more