Am in Bali right now and have done a fantastic trek up to Mt Agung- hard work but amazing. Booked a guide called Mudi (Mudigoestothemountain)- highly recommended. Also trekked rice feilds in Ubud by ourselves- easy, very interesting. And, have just done a trek in from our hotel in Candi Dasa by ourselves - about 5 hours up Gumang Hill and to the Gumang temple and down the other side and back into CD. about 6 years ago
If you get a chance to visit the UNESCO listed rice fields of Jutiluwih, it would be worth trekking around.
Another great trek is through the Munduk area - coffee plantations, clove fields, rice fields and some amazing waterfalls. Truly stunning place. about 6 years ago