These include such things as discovering, after a month of not doing much dissertation work, that you have already written an outline and several paragraphs of the chapter you were planning on drafting next. The gift of work you had forgotten you’d done. What a nice surprise.
Also worth being thankful for: land-grant college libraries. The ability to step outside your carrel and just grab off the shelf the book or the report or the extension publication or the government document you need. What a luxury.