Capri

There are several companies that organize boat tours from Positano to Capri. They include a visit to the blue grotto and probably lunch. They probably leave at around 9 or 10 and get you back to Positano by 5 or 6. You probably need an hour each way on the boat, how can you visit in just a morning?
Once in Capri, the boat excursion company generally leaves you alone for a few hours. You could take the chairlift to Monte Solaro and enjoy the view from the bar, take the funicular from the Marina Grande, go to Anacapri, to the piazzetta and spend a few hours on the beach before heading back.
there are also taxis.
I don't think you need a guide. It's not that you have a lot of historical sites where you need explanation.

You can easily visit Capri on your own from Positano (it's a very easy trip). However, I'd suggest the better part of a full day if there are several sights that you want to see.

You can buy tickets at the small booths down on the beach in Positano (which are on the way to the concrete jetty where the boats dock). When you arrive at Marina Grande (Capri), buy a ticket for the Funicular that goes up to town. As I recall, the ticket booth is on one side of the street, and the entrance to the Funicular is on the other. You'll probably find the prices (ie: restaurants) a bit on the posh side, which is commensurate with the ubiquitous posh shops (Gucci, Prada, Armani, Dolce & Gabbana, etc.). I didn't bother with the Blue Grotto, but there's information in the Guidebook if you want to visit that.

If you're going to take the last boat back to Positano, I'd suggest leaving lots of time to get back to Marina Grande as the Funicular can be packed in the late afternoon. I decided not to bother with the queue, and just took one of the open top Taxis down the hill. It was a very pleasant trip and I didn't have to deal with the crowds.

I have made the trip to Capri from Positano. Plan on the entire day. Decide what you want to see in advance to make it easier on you. Blue Gratto? Beach? Shopping? Or just a nice lunch and shopping. You can't do it all but choose and than plan for the whole day.

Thanks, all.