I bought a spring-loaded curtain rod at Ocean State Job Lot for $5 and then hopped over to the dollar store and bought these green baskets. I hung them with shower curtain rings. It's fun because you can pick out which baskets will work best for you--and not just for toys, you can use these caddys for shampoo and soaps and sponges and razors, etc--I found the best baskets don't stick out too far. Total cost = $8. There is no drilling and everything is adjustable.
Although sometimes all you need to have fun in the tub is bubbles.
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Dylan also loves trucks and some of them are absolutely huge. He has hundreds of them, some as big as three feet long and as small as two inches. When his collection outgrew our space I searched everywhere online for "truck storage" and found nothing that really worked for us. Most ideas involved baskets and buckets and square shelves. So I bought this 5-tier metal shelf at Target for $59. I painted a wooden sign which reads, Dylan's Truck Garage, and because he is an adventurous little boy I installed it inside of his closet so that I didn't have to worry about the shelves tipping if he tried to climb it.
I think I counted about 34 trucks/planes/helicopters/bulldozers/cars/buses tonight but the count varies depending on which vehicles Dylan is currently doting on. I use the higher shelves for arts and crafts storage since Dylan can't reach those anyhow. Because we call this his truck garage he is more than happy to park his trucks here at the end of the day. It feels less like cleaning up for him and more like play. And I love it because I can shut the door and hide three dozen very large toys.