We were there last week.
2/2.5 days is quite a short time, it will take you a two and half hours to drive from Picton to Nelson. On the way to Nelson, at Havelock, there is a nice icecream parlour (lots of different sorts) plus fudges made by owner. also the Mussel place - Had good reviews on Tripadvisor, but we thought it was a bit pricey, if a hot day I'd go for the icecream instead. Don't miss the Abel National Park. You can either get to the Abel Tasman Sea Shuttle from Nelson or Motueka - connects to Kaiteriteri - where the boat leaves from. We got the free bus from Mouteka to Kaikerikeri - and used the AbelTasman company. Good service, just phoned a few days before to book, went on their most popular discovery trip, a bit of sightseeing up the coast and then we were dropped off at Bark Bay, and walked to Torrent Bay, can walk a further bay if you wish. (Anchorage Bay), all details on their website: www.abeltasmanseashuttles.co.nz or free phone 0800732748, the walk is about 7km, plenty of time allowed before pickup by boat, so you could do a side walk to sandfly bay (it looked lovely) also Torrent Bay had a lovely beach.
-if you have time to go over the hill - suggest Pohara has a nice beach, just be aware that at the moment the beach is covered in lots of driftwood. Only negative is that if you want to swim, be aware that on the Tasman coast the tide goes out a long way and at some beaches can be quite shallow. Ligar bay and north also nice. (Pohara Beachfront Motel, lovely motel and hosts but you must book ahead, www.phoharamotel.co.nz freephone 0800 862 322, if you are looking for more budget accommodation, Moutere Inn phone 03 5432759 lots of craft beers on tap - which is the other 'don't miss' - if you travel one way up the main highway to get from Nelson to Moteuka, then come back via the Moutere Highway the other way - Moutere Inn is NZ's oldest pub - great atmosphere, we went back 3 times!!, good food - especially their pizzas - chef makes the bases himself. We enjoyed the salmon fishing farm at Takaka - they supply all rods and gear, you catch salmon and they will smoke for you, to eat there or takeaway. not cheap but nice. another nice place was Mapua - you can get the flat bottom fairy boat over to Rabbit Island (can also drive over to Rabbit Island - turnoff not far out of Nelson. Rabbit Island beach is nice, with barbeques provided, just take your food. again, beach has a lot of driftwood debri at the moment. or you could do both, take the fairy boat for a scenic trip (only $5 each) and also drive to rabbit island.
An alternative approach might be to drive from picton as fast as you can to Kaiterteri. Then catch a water taxe up the coast int eh Abel Tasmand National Park to a beach, spend the day there and either walk out or catch the taxi back at the end of the day.
Then you can head for the coast.
I would spend the whole day (or whatever time you have) enjoying the drive from Picton through the Queen charlotte Drive to Havelock and on through Nelson, stop off at Mapua for coffee (short drive from main road) through Motueka and either stay the night at Kaiteriteri or Marahau. Marahau is where the start of the National Park is so if you are going to walk out of the Park you will need your car there if you have to drive back to Kaiteriteri. Both places have cabins and back-packers or more expensive motels and Bed and breakfasts. Both have water taxis leaving frequently. Marahau is quite quaint because if it is low tide you are towed to the water on the trailer by tractor. There lots of combinations to go to the Park for the day between kayaking, walking and boat. The water taxis places will advise you depending on fitness etc.
If that leaves you another day - keep the same accomodation and go over the Takaka hill for the day. Lots of see on the way, Source fo the Riwaka River, Pupu Springs, try your luck at the Salmon Fisheries, stop at the top of the Hill to look at the caves.
The next day get an early start for the Coast.
Thanks everyone - this is very helpful!