Curtains. Even if you have blinds, curtains really helps dress up the place. They don’t have to be super expensive. Just something that looks nice from your local home goods store.
Hanging pictures is super easy. As long as they’re not huge, you can nail the picture hangers right into the drywall/plaster and it will hold up just fine.
If you want to spend a little more money, you can have large prints made of pictures and have them framed. The framing is the expensive part. I have a handful of pictures I’ve taken over the years, mostly of nature, that turned out really, really good. I had some of them framed and they are hanging throughout our house. I like them because I feel like it adds a personal touch.