He told supporters in Pennsylvania that he was keeping "one promise after another", dismissing criticism as "fake news" by "out of touch" journalists.
Mr Trump decided to skip the White House Correspondents' Dinner - the first US leader to miss the event since an injured Ronald Reagan in 1981.
At the rally in Harrisburg, the president said the media should be given "a big, fat, failing grade" over their coverage of his achievements during his first 100 days and told the cheering crowd he was "thrilled to be more than 100 miles from Washington".
He quipped that at the same time "a large group of Hollywood actors and Washington media are consoling themselves" at the correspondents' dinner "that will be very boring".
Until now, late president Ronald Reagan was the last US leader to miss the annual dinner, as he was recovering from an assassination attempt in 1981.
Turning to his election pledges, Mr Trump said the first 100 days had been "very exciting and very productive".
He said he was "delivering every single day" by:
- Ending "jobs theft" and bringing them back to the US from overseas
- Easing regulations on energy exploration, including halting a "war on coal"
- Pulling out of international deals not beneficial to the US, including the Trans-Pacific Partnership
Mr Trump also said the administration of Barack Obama had resulted in "a mess", stressing that he was ready for "great battles to come and we will win in every case".
On climate change, Mr Trump said "a big decision" would be taken within the next two weeks.
He earlier described climate change as a hoax, vowing to pull the US out of the Paris Agreement.
Ahead of the speech, tens of thousands took part in protests across the US against the Trump administration's stance on climate change. The People's Climate March was timed to coincide with the 100th day of his presidency.
In Washington, the demonstrators marched from the US Capitol to the White House.
Organisers said they wanted to put the climate debate firmly on the agenda for next year's midterm elections.