You can bring a small amount of cash with you if you feel like tipping the hotel staff. Within the hotel, they accept credit/debit cards and you can also have any extra charges like souvenirs charged to your room. If you do decide bring to cash, it would be best to exchange to Mexican Pesos before you leave the U.S./Canada as you're more likely to get a better exchange rate there than in Mexico.
If you're staying at Hotel Xcaret Mexico, the hotel offers complimentary Shuttle service to and from the airport at no extra cost. Just speak to hotel guest services to confirm your pickup time. There's also no need to rent a car if you plan on mostly staying in the hotel or going to the Xcaret parks and/or tours since there are shuttles that will drop you off and pick you up from the parks.
Yes. All the parks and tours/excursions are included with your stay. The only exception to this is the Xichen Deluxe Tour which has a government-mandated fee of $33USD.
Unfortunately, spa services are not included. However, Muluk Spa is really cool and if you get the chance, Diana and I recommend you try the hydrotherapy service.