Hi S.,
This isn't about appreciating what he has -- it's about learning that you can't always do what you want. This is a very difficult concept for a child your son's age to get. Some kids are more manipulative than others in trying to achieve their desires. Your son sounds like he prefers to take an emotional, passive-agressive route.
It sounds like you're doing a great job already, but you might want to try using logical consequences coupled with one reminder. For the example you used, here's a couple of alternative ways to handle it.
Before you leave, tell your son that the toy must remain in his backpack until you get to the beach, even though you know he will be very excited about showing it to his friends, and will have a hard time waiting (young kids--and a lot of older ones!--have very poor impulse control). Ask him to repeat this instruction to you, so that you and he both know that he's listening and understands the rule.
When you get to school, before getting out of the car, ask him to repeat the rule, and what the consequence will be for breaking the rule (see below).
Consequence option 1: If he breaks the rule, you will put the toy into "time out" and he will not get to play with it for a set amount of time after you get to the beach. This can be for 15 mins up to the entire rest of the day. This approach teaches him that your rules are not to be broken or questioned (assuming that is the lesson you want to impart).
Consequence option 2: If the toy comes out early and he loses it or pieces to it, the you will not replace the toy. This approach teaches him personal responsibility and the importance of taking care of his possessions, and is a better lesson in my mind. If he takes the toy out and is very careful about it, putting everything away before you go, then he has learned a very valuable lesson about possessions being valuable and not always replaceable. This is the first step to appreciating what you have!
Either way, the important thing is to stick to your guns and not give into the emotional blackmail he's using (sounds like a smart little guy!). YOU know you're doing the right thing because you're the adult. When he's 25 and his frontal cortex has completely matured, he'll know it too!
Thought number 2, which is separate from the discipline issue. If you want your son to learn about being fortunate, you have to show him that there are people who are NOT well off. Find a family care project to participate in. Have him collect gently used toys, books and clothes, as well as canned goods and take him to donate them to someplace like a homeless shelter for families, a women's shelter, or a church/synagogue/mosque with a needy congregation. These types of places are not overwhelming for a young child to visit, and they do get the idea.
I remember when my son was 3 and we were flying home from an international vacation. During the trip, we had visited a sugar cane plantation in the Dominican Republic. Talk about poverty! Most of the children were dressed in rags and were pretty thin. On the plane home, I was reading an article on the, at that time, famine in Somalia. It had some heartwrenching pictures of skeletal children holding up empty food bowls. I didn't realize that David was looking at the pictures until he asked about what the children were doing and why they looked so hungry. I explained to him that they didn't have any food to eat where they lived. Shortly thereafter, our meals arrived. David looked at his dinner and asked me to send it to the children in the picture. I explained to him that we couldn't send that food, but we could send money to people who could buy food when we got home, and we could take food to people in our own home town who didn't have enough to eat, too. When we got home, I had him go around the house and look for loose change to put in our charity box. I also gave him money for doing little tasks and encouraged to share some of it with the box each time. There was never an arguement -- he loved to put money in the box. When it was full, we counted the change and I took him to the bank to change it into cash. He very proudly told the teller that we were giving the money to the children in Somalia so they could buy food. At home, we counted the cash and I wrote a check against it, then we took it to a local Red Cross office and he asked them to send the money to Somalia. At the same time, we collected clothes and toys, and sent them with another check to a person in the DR who helped out the people on the sugar cane plantation we visited. We also got together with a family group we belonged to and started preparing meals once a month for a local men's shelter. David loved to help cook the meals and was delighted to come to the shelter to deliver the food and talk to the men (under my very watchful eye, of course!)
Over the years, we have either participated in or instituted many, many such projects, both locally and globally, through our synagogue, the kid's schools, scout troops and on our own. Both my kids have developed wonderful, generous spirits and now seek out opportunities to do volunteer work.