If you know what you're mothers favourite perfume is, then buy her some! Or just some she might like. It's a common present but I think it's an easy but nice one to get. It makes it alot easier if you know what scents she likes too.
Get her some jewellery! I think jewellery is such a beautiful and sentimental present and you can't really go wrong. It can be gold, silver or bronze and can range between necklaces, earrings or rings. I'm sure most mothers wear jewellery so this is a lovely gift idea.
Your mum may not be into the beauty side of things but maybe she likes skincare. If she does, it would make a nice present and she'd probably love it! Take note of what products she likes and what brands, see what she's running out of and maybe experiment a little by getting her a few of your recommendations!
If she doesn't like beauty or skincare, she might be interested in haircare! Have a look at what she already owns, see what she is into and think outside of the box. There are so many hair products and brands out there, so pick wisely!