I've been (as has almost everyone in WA). Rafting was fantastic. I would pass on the Tat unless you are some what of an expert. A few people have died from dodgy tats in Bali.
Waterbom Park (not sure on spelling) is great, as is the Safari Park. This is a world class Safari Park so make sure you go if you haven't been to one before.
Get yourself a guide and let them help you organise everything, our's was very helpful. We also went out to see the Volcano, the silver smithing and a traditional family home in the country, well worth the trip.
It all depends what you are after.. if you just wanna drink and party there's plenty of that as well (not really my thing).
Again, I would pass on the Tattoo idea, cheap is not always good for something like that.
Enjoy!