i-to-i is a good volunteer company.
You can search by country or volunteer project type on there I think. Think of about £450 fo 2 weeks and then £100 to £150 per week after. Accomodation and food included.
Projects are usually only monday to friday with weekends plus one day off every 2 weeks / month for you to do what you will in. Bear in mind, though, that on a rainforest project you may be way out in the sticks and so have no real chance of doing anything / going anywhere even on your free days.
There is plenty of variety on environmental volunteer projects (you could be working in the rainforest clearing pathways one day, going out counting animals another, giving tours on another and if you're near "civilisation" so to speak, teaching the local community about the importance of being enviormentally friendly the next!), however, you would need to bear in mind that you would be there to do a job not to travel and see lots of different things.
I think you can have a lot of fun volunteering, just don't expect too much freedom or the variety you'd get from travelling round a country.
I hope you have a lovely time whatever you do.