Florence airport

Joe,

I would presume the taxi ride (airport to Siena) would be very expensive since you're talking about a 50 mile ride.

Another option would be taking a taxi from the airport to Firenze Rifredi train station (around a 10 minute ride), then taking the train directly to Siena from there. Once you arrive to Siena, you can take a taxi to your hotel.

You may check train schedules on trenitalia.com

I've not rented a car in Italy, so can't help you with your car rental question.

Have a wonderful trip!

Edited to add...

You may check bus schedules here.

I would like to know the cost of a taxi ride for two people with luggage from the Florence airport to Siena's city center.
About 120-140 Euro

I would also like to know what are the bus times for the Rapida buses for the same route on Sundays.
http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/dae6e4ed0b91112bf1ffa41835f96ce6.pdf
(see last table in the PDF=Festivo=Holiday schedule)
Buses depart from the bus station across from the train station, downtown Florence.

Also, how difficult is it to find and return a rental car at Florence airport
A bit tricky, just set the GPS for "Via Palagio degli Spini"

and do you need to shuttle from the rental car drop off and the airport.
Yes they have a shuttle bus. It's very close as the crow flies, but there is the freeway in between, so it would be a long walk to go around that (and I'm not even sure there is much of a sidewalk)

From the airport you may want to consider the train option from Rifredi as Priscilla mentioned.

Thank you for all the helpful info!!!!