I've never done any of the structured rides. I always go on my own. There are plenty of bicycle-rental shops in Ubud town that will rent you a bike for very reasonable rates. I can't remember what the rates were specifically, but they were insignificant-- on the order of $5 or $10 for a whole day, no security deposit required. The bikes are usually 18-speed mountain/road bikes and they're usually in pretty good condition, but be sure to test the cables for smooth gear-changing. The rental shops typically DO NOT have helmets, so you should ask your hotel concierge if they have any for borrowing. I have a strong sense of direction and can now get around fairly well without a map. But even a first-time visitor can do fine on a self-guided tour with a map, as Bali's topography dictates the placement of roads and you can tell which way you're going just by looking at the slope of the road (upgradient generally runs north, downgradient south, cross-gradient east or west) or at the sea or volcanoes. The things to be most careful about of thick traffic in population centers and heavily laden trucks zooming by.