Opposite of boondoggle

It's often called a busman's holiday

A vacation during which one engages in activity that is similar to one's usual work.

American Heritage

Collins suggests the term derives from the idea of a bus driver taking a driving holiday

It's a rather boring and obvious answer, but what I've heard it called is working vacation (USA) or working holiday (UK)

a trip, sometimes to another country, on which you work

There is also the slang workation.

A workation is similar to a vacation where you can take your work along with you.

Depending on your job and tax bracket, a pleasure trip where you squeeze in some business might be a tax write-off vacation.

[T]he IRS says that you can deduct expenses for taking a business trip. There is no reason the trip shouldn’t coincide with your next vacation. (Bill Walston, real estate blog)

A fellow . . . needed insurance. Problem was, he was in Branson, Mo., and wouldn’t be back to Iowa before summer. “No problem,” I told him. “Mary and I have been champing at the bit to go to Branson. We’ll meet you there." . . . Done! A tax-write-off vacation. (Curt Swarm, Newton Daily News, "My tax write-off vacation to Branson")

The line here is a bit blurry (though maybe not to the IRS), so this term might also apply to the original situation described (a pleasure trip only masquerading as business).