I want to spend the day at Cozumel (staying at Iberostar Paraiso) exploring the east side places. We'll be taking the ferry from Playa Del Carmen. The thing is that I'm scared to rent a vehicle and drive there. I was there last year and the city streets seemed chaotic with taxis zooming around. What are other options and how much? Thanks.

There really are no other options. A taxi trip round to the east side and back will easily cost over $100. The scooters are even more dangerous than renting a vehicle.
It seems chaotic but it's not that bad,

I am not familiar with Cozumel, but I have spents lots of time in various cities throughout Mexico. I've never been anywhere there has not been good bus service. Is this an option?

Usually buses are excellent in Mexico, but I don't think you can really take a bus around Cozumel. The other side of the island isn't really populated. I think you'll have to try to rent a car. I think once you get outside of San Miguel, you'll be ok. Mind you, I've been to Cozumel several times and we've never yet rented a car, so I'm probably not the best to give advice!

We rented a car in Cozumel and it was during "Carnival" but we didn't have any problems except with parking. They have a nasty habit of towing illegally parked cars away for a big fine in San Miguel. Lesson: Read the signs!

A 3 hour taxi tour around the island with the driver waiting for you at various stops will cost about $50 USD. Its negotable. For accurate taxi fares in Cozumel see: http://www.cozumelmycozumel.com/Pages/Taxis.htm

As stated, traffic is quite sparse once you get out of town. Traffic in town isn't really bad except right on main street and the few surrounding blocks. Parking is always a problem.

If you just want to shop in San Miguel, you can walk everywhere. The ferry pier from Playa is right downtown. If you want to snorkle at Dzul Ha or other beaches, just have a taxi drop you there and come back later to retrieve you. Or do the island tour for $50, which is much cheaper and easier than renting a car for one day.

There is no typical bus service on Cozumel.

Hope this helps.

Rent a jeep. You'll be pleased that you did. You can take it at your own pace in town. We really enjoyed the trip round the island, stopping to swim at deserted beaches and having a drink at Bob Marley's and Mescalitos(sp?) at the end of the Island - fabulous grilled shrimp. We also toured the ruins and seemed to be the only people there.

I agree that renting a jeep is the best option. You can go at your own pace and enjoy the sights and beaches and move on when you are ready.
Should you not be comfortable driving yourself (although it is easy once you're out of town, and impossible to get lost) as TC said, you can get an island tour with a taxi driver for around $50.00.
DEFINITELY do not go the scooter route. We've seen so many accidents over the years - one fatal.
There is no organized bus service, so the jeep (or car) is your best option.
We usually spend $50-60 with Hertz or Avis through the hotel where we stay.