Although I'm 100% in favour of clicker training, in this case it might not be the best approach to start with.
What you want to do is to make the dog enjoy and initiate playing fetch. To me this is closer to classical conditioning than to operant conditioning.
Clicker training, and positive reinforcement training is about operant conditioning, you condition you dog, through positive reinforcement, but the dog has to think his way to get the reward. Subsequently he "figures out" what actions are "good".
Classical conditioning is about innate responses to stimuli. It is in a sense more "primitive" and bypasses any kind of reasoning the dog might have. When you take its food bowl he just gets hungry, there is no reasoning involved. This is opposite to the operant conditioning where you make the dog think, eg. for all the basic cues (sit, down, etc.).
In a way you want to play fetch with your dog so that he enjoys it. This is not to say that dogs don't enjoy what you train them to do, but here we want a very "primitive" reaction, you take the toy and the dog, without thinking starts playing.
In addition it seems that you tried to clicker-train him in an environment with a lot of distractions, that might be the reason why he didn't respond to your clicker. Whatever the real reason, if it doesn't work, that means you have to "go back a step", either in term of difficulty, distraction or attention (too long training session for example).
So how do I answer your question with this ?
You should classical condition your dog toy = fun = excitement. You mentioned that he wandered off sniffing. Take him on leash, try to see where he want to go sniffing, unleash him, and as you let him go, throw the toy. Doesn't matter if he goes to the toy at this point. Ask squirrels for help, once the dog sees them and is ready to dash to them, throw the toy. Do the same we he gets out of the car at the park. You can do the same association with food. First "throwing the toy = food", then "throwing the toy, the dog runs = food", etc. You got the idea.
If you do this, maybe 10 times, you'll have the dog conditioned and excited as you throw the toy. This will then be a perfect starting point to move on the operant conditioning, as you did with the clicker to a real "play fetch".
Try also to set up clicker training sessions (for whatever behaviour) in more distractive environments. To teach him "play fetch", or more generally for outdoor training sessions, try to use the distractions and the environment at your advantage. Use it as a reward. Fetching the dog, OK you can go sniff on your own, etc.
Always make sure that the dog can succeed. For the first part (classical conditioning) that means throwing the toy only if the dog is somehow excited. Not at the end of a long walk. For the clicker training part, don't do it if the distractions are way to high.