Yes you can but you will need to be organised!
The helicopter and Skybus (small fixed wing plane) services will get you there fast but in the season they are are often full so book early! There's greater availability on the Scillonian III which sails from Penzance between mid-March and the end of October. It usually departs from Penzance at 9.15am with the return sailing leaving St Mary’s at 4.30pm. Departure times can vary depending on the tide and weather. This is the least expensive option but as the journey takes about 2¾ hours (you get a scenic view of the West Penwith coastline on the way) you'll only have 3 to 4 hours on the islands. That is enough to walk around the Garrison on St Mary's or take lunch (Juliet's Garden at Porthloo is a good mix of walk and lunch but also consider Longstone Cafe and Carn Vean Cafe).
In the busiest periods, there are sometimes two sailings a day, one departing Penzance at about 6am and one at 1pm. A special low price is usually charged. The return sailings from Scilly are at 9.30am and 4.30pm. If you leave on the early sailing and return on the last sailing you will have enough time to visit one of the off-islands (the best of Scilly). Tresco is a favourite destination for many. You might consider sailing over and returning on a late Skybus flight to get the most time out of your day – there is usually a special deal – book both on the Isles of Steamship Company website.
Skybus flies to Scilly all year from Land’s End Airport. You can also fly direct to St Mary’s Airport from Newquay and Exeter airports; this is a more seasonal service. The flight from Land’s End Airport takes about twenty minutes, from Newquay about thirty minutes and from Exeter about sixty minutes. Penzance Helicopters operate a helicopter service from Penzance Heliport (near Sainsbury’s as you come into Penzance on the A30) to St Mary’s Airport and Tresco. The flight time from Penzance Heliport is about 15 minutes. These are more expensive options but will give you the most time on Scilly. If you get your timings right you could get to an off-island for a day or just walk from St Mary's Airport around the north of St Mary's. The inter-island ferries leave right next to the Scillonian berth, the skipper will let you know when and where to get the return ferry to be in time for departure back to the mainland.
Don't foget our maps and guidebooks to plan your day.