When you're out and about in the daytime, you can be as casual as you like, e.g. cropped trousers, t-shirts, perhaps a vest-top with a loose cotton shirt worn over it, so your neck and shoulders don't burn.
I am not a dressy person either and have found white cotton trousers are great for changing into in the evenings, with a selection of pretty, lightweight tops or tunics. Wear flat shoes or sandals if that's what you like, I haven't worn heels for nearly 50 years, and I've had several holidays in the Canaries!
Some hotels have a more formal dress code than others, but women rarely have a problem, it's the men - the more formal hotels may insist that men don't turn up to evening dinner in sandals, shorts or vest-tops. Others are more casual - have a look at the hotel website, or Trip Advisor, if you have concerns.
One hotel we went to in La Palma had a notice in the rooms asking guests to be "elegant" for dinner. Well, too late, we couldn't go home and re-pack. We just had a giggle about it and decided we couldn't manage "elegant" but would aim for "clean"!