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Tenerife, at the right time of the year you can have a snowball fight in the morning and then sun bathe and swim in the afternoon. Its the biggest of the Islands offering a multitude of activities, from climbing to scuba diving. That time of the year it will be very warm, 25c+ it is a little colder in the north, at that has more rain, the reason they grow more vegetables, in the south they grow Bananas.
There are two smaller Islands off the south coast of Tenerife El Hierro, and La Gomera they are both great places to visit,
I do no think that you can guarantee sunshine, as the weather is changing throughout the world, but you have a great chance of having fantastic weather.
Have a good time. and have a Dorada for me.
RR