Spain in winter

You could start researching Andalucia

Costa del sol.

My wife and I spent a month in Malaga in Jan/Feb a few years back. Sunny every day but one. Typically high temperate was 70-- the sun made it feel warmer to us. Malaga has a few museums (a picasso museum, a car museum etc), a nice ruin (Alcazaba ), miles of walking by the sea, ferries, trains, a good airport and of course buses. We took spanish and tango courses and loved it. We stayed minutes from the cathedral-- nice area, quick access to sea, restaurants etc.

I believe RS has a nice write up on Nerja-- a town not far from Malaga (which is actually a good sized city.) Marbella also close is more upscale. The whole costa del sol is touristy but still charming although not in the same league as Sevilla for sites.

Timeanddate.com is superb for past weather, humidity, wind, etc. I am a faithful user before planning trips. It goes back at least 20 years.
I have been to Spain twice and love the country, especially the Andalusia area. Our first trip was in early April 2019. It rained sideways in Madrid with flakes of snow that never settled. From there, we went to Seville which was sunny and in the 50s. On to Granada with much rain and in the 40s. Barcelona had so much rain that our tickets to the top of the Sagrada Familia were canceled. We couldn't even see the outside well due to the rain, but there was that one day while visiting Park Guell that was in the 60s and sunny.
This January, we made the same trip with beautiful, sunny weather in the 60s. I don't think you can count on the weather. But, not even the rain in Madrid stopped us in 2019--we rode in the top deck of the HOHO bus and let the rain just rip! I hope you love Spain as much as I do.

Canary Islands

While you may (or may not) have pleasant, sunny days on the Costa del Sol at that time of year, it is not likely to be beach weather. Water temps in the Med will be cold.

I agree with Scudder, do what the northern Europeans do and head for the Canaries in the winter

Is tenerif/canaries safe and enjoyable knowing there were fire recently