We could start with all the things the federal government wasn't involved in prior to the enactment of the income tax.

You're not answering the question.

The world has changed somewhat since 1894.

Maintaining over 700 military bases across the world, endless war (handouts to Military-Industrial-Congressional Complex companies), corporate welfare, foreign aid, public art, Africa, etc...

OK, that's a set of specifics, though for the first you might find much of that has some justification (economically, not necessarily morally) via geopolitics of both raw materials (petrochemical and the Harbord list) and trade.

FYI the last three items amount to about 2% or less of Federal spending. All discretionary spending was 16% in 2016:

https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/U....

Excluding entitlements (Social Security, Medicare, debt service) it's still minuscule:

http://memepoliceman.com/wp-content/uploads/2015/09/viper-re...

You asked what the government could spend less on. I answered the question. What more do you want?

You've not given specific cases, and your hand-wave ignores reality.