What to do with dead times during your PhD? I am about to finish a phd in economics (couple of years left). I am in good shape: 


I have a nice on going solo authored paper 
I have a co-authored paper submitted to a journal and waiting feedback
One other large project on-going with another co-author
I starting to work on a new project that requires heavy computation.


Now here's the thing. At the moment I have a lot of dead time. 

Paper (1) - I do not feel there is much I can do now. I am presenting the paper in couple of places in the Summer and hope to get feedback. 

Paper (2) - We are waiting for referees to come back to us.

Paper (3) - I am waiting to discuss with my co-author next steps - he is on holiday for the next 3  weeks.

Paper (4) - I have setup what I could on this paper. I am running preliminary results (but the codes take up to 4 days to run). 

The key question is what to do in the meantime. Of course, I could work on paper (1) a bit more, but I feel that productivity is low on that one at the moment. I could start something new as I have some ideas on the pipeline, but I am avoiding this as something might come up unexpectedly on papers (1), (2) or (3), and giving a false start to a new project might not be optimal. 

Edit: A suggestion below was to learn a programming language. However, I am "fluent" in Fortran, Matlab, Python, Stata and SAS ... and I have no use at the moment for other languages. 

I could always read papers, but how much can you read if do not have a proper goal in mind?

Any advice?
The best thing to do in the summer after your "third" year of PhD is to take stock of where you are and where you want to be. Then you want to create a plan to get there - basically, write yourself a realistic roadmap, working back from what a successful job search looks like. 

In Economics, you will be focused on the job market in year 5. You will want to be prepared to convince committees that you are a good hire - that you are prepared to hit the ground running and get tenure at their institution. Look at the CV of the hires from this round for your ideal institutions and compare your CV. Do you need another paper? A grant? A major award? Make a list. 

Then create your calendar. Put on important deadlines. Plan when you want to submit other things and what the turnaround is likely to be (it will be long). 

Keep in mind that the fifth year is basically a wash. You don't achieve anything of note. You may finish an R&amp;R, but other than that, refining your job market paper and your presentation plus applying to and interviewing for jobs takes all your time. If you are not clear on what that year will look like, read some of the excellent job market guides from AEA or Econ bloggers. Also consider the book "The Professor is In" or Vitae columns. 

Given this, I would probably argue for thinking hard about grants and what proposals you might want to have ready to send during year 5. This involves some real generative, creative work. It means thinking about what you want your research agenda to look like as an Assistant Professor. Having this sort of thing in the pipeline will be impressive to hiring committees.

Once you have done this, you should also consider doing things that make you happy. See some family and friends and get some exercise. Or do something vaguely work related but actually pleasant like creating your website or signing up for some summer institute/workshop. This summer is the last time you will have to really kick back (presuming you are worrying about moving after you graduate)!