Josh L.
Google
You can spend 5 minutes or a few hours here, it's up to you. I ended up spending a few hours, myself. I also stopped at the Lake Mead overlook before getting to the Dam, which was a cool view!
There's a small free parking lot before you drive down the hill into the main part of the Dam that you can park at to walk up onto the bridge overlooking the Dam. It's worth the few minute walk out onto the bridge. It's really windy though, so hang on to your stuff.
As far as parking at the Dam itself, it's $10 at the time of writing this to park in the garage on the Nevada side, but if you drive across to the Arizona side there are several lots along the road going up the hill that are all free. I believe it's Lots 11 and up that are free. I parked in Lot 12 and it was just a few minutes longer to walk down to the Dam as it was from the parking garage. Worth it to save $10 if you're able and willing to walk a few extra steps in my opinion.
I bought a ticket for $15 to go on the exhibit self-guided tour, which was ok. It was really cool to watch the 10-ish minute film on the history of the Dam and the exhibits were also interesting, but it doesn't take long to go through them.
I kind of wish I'd gone on the power plant tour too, but ended up only doing the exhibits.
There's a small gift shop at the top of the hill on the Arizona side that I feel like most folks miss unless they're on a guided tour in one of those little vans. There's not a ton in there, but there's some interesting alien / Area 51 and additional Hoover Dam paraphernalia. It was about the same pricing-wise as the main gift shop with slightly different stuff.
It's honestly pretty wild that people built this, especially with the tools they had at the time. Overall definitely worth a visit!