If you have a few hours to spare and you're looking for a scenic spot to have a picnic lunch, head to to Lake Matheson for a quick and easy short walk where you can enjoy nice views of the Southern Alps, reflecting in the lake's waters. The lake is a short distance from the Fox Glacier Village.
The Pancake Rocks (Punakaiki) are definitely over-hyped in my opinion & definitely not worth the trip north from Greymouth unless your'e enroute to Nelson / Abel Tasman / The Golden Bay area anyway. That being said, it can be a nice spot to break the long ride from Greymouth to the northen regions of the island. The small reserve will take about 1-2 hours tops to explore and most impressives sight, are the 'pancakes' themselves (obviously) - interesting rock formations, right on the coastlne that were carved by the endless gushing waves of the Tasman sea.
It might be considered by some folks as a somewhat tourist trap, but I don't care - I still liked it. I'm talking about Puzzling World of course. This half-musuem, half-playground-for-adults contains a wealth of illusions that are meant to mess with your head. I won't spoil the fun and tell you exactly what to expect, but overall - the experience was very positive. You can also pay a little extra and try to work your way out through a giant maze (which I didn't really care for, the "musuem" part was much better in my opinion).
After we finished with the glaciers area, going pass the Copeland track and some typical west-coast scenery, we decided to stop for a little while around Haast. The beaches around the Haast area were pretty much empty when we were there, and there are several options to go around. If beaches is indeed what you are after, I wouldn't recommend going on the Haast-Jackson Bay road since you virtually don't have any beach access until you actually reach Jackson Bay itself. Instead, opt for other places such as the beach next to Ship Creek - about 16 km north of Haast itself. As always, the Tasman sea will be absolutely freezing cold - but when did this ever stop people from running butt-naked screaming into the water? A few hours to a day should be enough to explore the Haast region including making some stops & short walks near Haast Pass, which arguably sits on one of the most spectacular stretches of road in NZ's southern island.
After we finished with Nelson Lakes, we continued south and suffered a few days of heavy downpours while passing through some forgettable towns along the west coast. We ended up in Westland National Park, home to the popular Franz Joesf & Fox Glaciers. The glaciers, going down all the way from the slopes of Mt. Cook (& its neighboring peaks) area to the coastline are remarkable and worthy of your time. There are several options to explore the area, including heli-skiing and heli-hiking. If you are a backpacker (like me) with little cash in his pocket, the cheaper option would be to explore the lower regions of the glaciers. While it may not be as clean & pristine as the upper edges of the glaciers, the views from the mid-bottom area are still beautiful and the whole experience is quite impressive. You can go with the commercial tour guides that are located in the villages next to the glaciers. They will take care of everything you need so you don't need to have any ice/glaciers skills at all - but as i said, these are commercial tour operators and if you dream of a intimate adventure in the glaciers by yourself - forget it. The alternatives are either going up by helicopter (expensive!) or going to explore the glaciers by yourself independently, provided that you are experienced enough with glaciers or mountaineering and equipped with crampons, ice axes etc.