A day trip on the Scillonian to the Isles Of Scilly is a wonderful way to spend the day. Do be aware though, if the sea is rough, it can be a choppy ride! On a beautifully calm day though it is the most amazing sea trip.
At the time of writing this, the Scillonian leaves Penzance harbour at 9:15am and lands on St Mary's at 12noon. You disembark and get until 4:30pm to visit the island. The boat leaves St Mary's at 4:30pm and gets back to Penzance around 7:15pm.
Some days the boat leaves later and comes back earlier, so again you would need to check times in order to give yourself a nice amount of time on St Mary's.
You can go for a longer period of time over to The Scillies, but prices do go up a fair bit. You can also get the helicopter or plane over too.
Car Parking
You can't take your car on the Scillonian, so you need to park it somewhere in Penzance. The Scillionian company do offer a park and ride service from Trereife House, but you should be able to get parking at Penzance Harbour council car park much cheaper for the day.
Please check the Scillonian website for up to date sailing times and ticket prices.