[CyberFonic]

I'm surprised that your manager hasn't assigned specific tasks to do.

If I were in your position, I'd take some holidays. That would be the best way to recharge.

If you still have to go to work, then brushing up on some skill in an area that will help you be more productive in the next project is probably a good idea. That could be increasing your proficiency in a language, framework, libraries. A small side-project to automate some rote task could also be very beneficial.