What is the best way of joining multiple pieces of wood together?

I’m looking into building something for my girlfriend, and I’m trying to figure out the best way of joining the wood. There are several (between five and seven) pieces of lumber.

I’m using neither super thin / super durable wood. They’re the slats from some pallets that I had in the back yard. I’m hoping that I can join them with something I’ve got out in my workshop (nails, screws, wood glue, etc comes to mind, for me anyway)

If it matters, I’m using the wood as my canvas as an art piece for her.