These are a few I have been using recently. There are definitely more posts about app suggestions that you could find, but for now, here are 4 apps that I suggest:
Google Street View
-You may have already downloaded this, but this standalone app is a great way to explore the world in VR. I have used this app the most since I purchased my headset.
Cardboard Crash - Sundance Edition
-This app is about the ethics of smart car AI, it is pretty short, but a fun and free experience.
Squarepusher
-This app is a trippy experience with music from the electronic artist squarepusher. It has really interesting aesthetics and is also free.
Wizard Academy - VR Cardboard
-This is a game, but I think it is an interesting technique for VR. You basically have a wand that is like a laser pointer as your cursor, but how you move is by walking in place. It is full of mini games for your "wizard training" with low poly styled graphics.