On my second week in Japan I went to Osaka and had to visit one if its main landmarks, Osaka Castle.
Access to the castle by Rail is fairly easy. Jr trains run a loop line that circles around Osaka and the easiest stop for the castle was Morinomiya Station. I got off at the station and across the street was the entrance to the Castle/Park.
The Castle park is clean and great for walking, especially during warm/cool weather.
There a few different paths around the park but I had familiarized myself first on which path to take and I'd recommend you do the same if visiting as it could mean a long walk in the wrong direction.
The castle itself was very beautifully carved out in the landscape. So many great photo opportunities with the greens of the trees at the forefront of the castle.
There's also a nearby cafe you can sit and take it all in after a nice walk.
To go inside the castle costs 600 yen and you can also wear a Kimono and Helmet inside for an extra 300 yen. Unfortunately I arrived late (last entry is around 4.30pm) so I didn't get to try on the helmet and outfit.
Id recommend to anyone to visit Osaka castle, especially if its your first day in Osaka. It's a nice relaxing way to spend a first day in a big city.