Im heading to Koh Tao in two weeks for 5 days and my partner is planning to do a dive course that im not realy interested in - so what else is there to do in Koh Tao?

There is not a great deal to do apart from diving, snorkelling and sunbathing. Take a few good books!

There is water skiing, wake boarding, kayaking, banana boat rides, even sailing lessons. On land you can go trekking, mountain biking or free climbing. There is island style bowling and petanque.
There is Tai Chi, Chi Gong and Yoga, Reiki, Foot Refelexology, all practised and taught.
And if you wish to explore the island in depth, especially on foot, there is a detailed, topographic fold-up map "Koh Tao in Focus" available for free in shops, resorts, and dive centers.

Quite enough to keep you busy for five days......I think!

Wow! it must have changed a lot since I was last there about 5 years ago. Is it still full of backpackers on gap years?

Ohhh that sounds like fun - Any particular organisations that run such activities that you can recomend?