Huge, beautiful park with a lot of neat stuff to explore. We didn't have a whole lot of time to spend--we climbed up to Bao Thien Tower and (practically next to it) the Ho Chi Mihn Temple. After we came down we checked out the ruins of the French stone church. So all together, we were there for a few hours.
The climb up the big hill to the tower and temple involved many, many stairs. But you are in the shade the whole time! Still, if you have mobility issues or you hate stairs, this one might not be for you. But it was really great once you reach the top! There are other people on the path with you, and there are plenty of benches and other places to stop and rest if you want to. I recommend bringing water.
The park is full of beautiful flowers. Everything is so lush and green, and you hear constant bird song. For the record, we did NOT use our bug spray, and we had not trouble. No bug bites.
The park is a 90 minute drive (60 km) west of Hanoi. We booked a driver with Golden Holiday Travel. He met us in the lobby of our hotel and took us straight there--no difficulty at all. Cost was $75.
Admission cost: 60000 VND ($3) per person. I think your driver is free. There is a ticket booth right at the entrance to the park. Our driver stopped---we ran over and bought tickets. Super easy.