unless you're into prospecting or meteorite hunting - in which case there's a lot of opportunities in the Southwest US on BLM and other federal land. https://www.detectorprospector.com/ is a decent resource
I've been doing it for about 8 years. The best part for me is the research. Going through old maps and topographies and aerial photographs is good fun for me.
The hardest part is getting permissions from land owners, many of whom have no idea what you're trying to do and are therefore rightly suspicious. But I'm fairly outgoing and friendly in nature so that hasn't been too difficult.
As others have mentioned, if you're in the US, you're unlikely to find very old things like European and Asian hobbyists are able to, but finding 150 year old coins and war relics is satisfying enough for me.
Best way to get started would be to watch The Detectorists on Amazon.