If you are on holiday in Gran Canaria but you fancy visiting Tenerife for the day you can either fly or get the ferry. Flights are about 40 euros and there is ferry information here: http://www.aferry.es/Las-Palmas-de-Gran-Canaria-ferry-es.htm
You can get the hovercraft or the slow ferry, depending on your budget and how many hours you want to spend travelling. I’ve been across on the fast ferry and it was about 86€ for 2 adults and a car (it is much cheaper if you don’t take a vehicle).
Tenerife is home to Loro Parque, which is a really good zoo, but to be honest the island isn’t that much different to Gran Canaria and if you just fancy a change of scene you are better off looking at renting a car in Gran Canaria and exploring the island you are on.