I have a conference in Cannes July 5-8 and will be arriving in Paris on the 3rd, working my way toward the coast with plans to be in Cannes by 16:30 on July 5 for a dinner engagement that evening. On a couple of the days, I'll be free after 16:30. Would it be worth it to hop a train to go somewhere like Antibes or Eze for some late afternoon sightseeing and dinner?
You could but it would be a bus. Personally I prefer Nice over Cannes and you could take a train but it is a short hop and the bus would work better. And Villefrance on the other side of Nice is worth an afternoon stroll. Nice little beach there and a bigger beach in Nice. However, the beaches are gravel and not fine sand. Now are working you way to the coast?
Taking the train from Cannes to Antibes works just as well as taking the bus. Trains run 2-3 times/hour and the ride takes 10 minutes. Walk around the walled old town and have dinner.
Going to Eze-le-Village won't work for you. You would have to leave Cannes about two hours earlier to have any time in the village before catching the last bus back to Nice.
Take either the bus or the train to Nice, explore the old town (Vieux Nice), and have dinner.
Frank, I'll be taking the train from Paris to the coast.