How to efficiently plan holidays in academia?
I tend to combine my holidays with academic travel. In part, because it reduces the cost of going to conferences: I don't have to fly half way across the country just for two days. In part, it also reduces the monetary cost of holiday travel.

For example, adding 2 days for sightseeing/relaxing after a conference doesn't impact flight reimbursement. You pay for the hotel/airbnb, but that's it. That is especially appealing if you are going to a conference in Europe [US] and you're based in the US [Europe], such that the flight is a substantial part of the cost.

I've also gotten flights to other cities reimbursed. As long as the flight X -> Y -> Z is not more expensive than X -> Y -> X, neither my university nor others that have paid for my travel have had any objections. It almost always costs them less (and never more), so it's a win-win situation. In that case, I still end up paying Z -> X, but that's half the fare I'd otherwise pay. If the flight to Z costs more than to X, I let them know and claim less than the actual cost for reimbursement.

Another advantage of this is that it spreads out vacation days throughout the year. I never feel guilty taking off a day here and there and I'm substantially more productive afterward. Doing nothing for two weeks doesn't particularly appeal to me and would likely just stress me out once I got back.