There are plenty of day trips available that will get you to the ticket office for the ascent of vesuvius, and these are often combined with e.g. a trip to Pompeii (although I would advise taking a separate half day trip to Vesuvius, as the ancient sites such as Pompeii require a whole day to explore).
Once you reach the ticket area, you can then hike up the path to the summit, which takes about 45 minutes. There are vendors from which you can hire a wooden walking pole, which helps, but is by no means essential and on the way up you can buy a drink if you didn't pack one.
At the top, you get great views over the bay of Naples (weather dependent), and you can peer into the great crater. There is no volcanic activity, but I felt it was still worth the hike if only for the views and to say that I'd been to the top of a famous volcano..!!