Hi, I just come back from Cancun this Monday.

Instead of stay at Cancun, you may stay at another place call 'Playa del Carmen'. It is because :
- It is distance from Cancun airport for 1 hour. You don't need to go into the Cancun downtown.
- It is a beach with many tourist restaurant, souvenior show, hotel, etc.
- It is the place to take ferry to Cozumel.
- It takes 1 hour to Tulum by bus. (I think you are target to 'Tulum Reins' right ? You may take a mini-bus and stop in front of the entrance.
- It takes 4 hours to Chichen Itaz.

For Chichen Itaz, I am not sure you can take the night view or not because the park is close at about 5pm. For me, I stay in Playa del Carmen and take a day-trip to Chichen Itaz by bus.

You will find that Playa del Carmen is more beautiful and less expensive than Cancun.

If you need any info, please let me know.

Hi, I try to answer what I know.

1. Yes, Playa del Carmen is a safe (of couse, some basic sense is required) and convenience place, both for restaurant and souvenior. (In fact I think the souvenior in Cancun, Cozumel, Playa del Carmen are similar).
2. It's up to you to go Xel-ha or Xacret. I haven't any comment.
3. For the 'night show', I think you need to join the local tour and stay near Chichen Itaz. The distance between Chichen Itaz and Playa del Carmen is about 4 hours. There is not bus to leave at 8pm (except those by local tour).
4. For Tulum, inside the Tulum Reins, there is a beach which is beautiful. But I agree that you may not spend for 1 day. You may take the ferry to Cozumel at 6pm (and hence get more time for relax in Cozumel).
5. If you don't have dive license, you may get a 'discovery' dive tour, or get an 'Open Water' dive license at Cozumel. It is not very expensive.

Personally, if you have about 8 days, don't just spend in the beach. You may visit other cities in Yucatán Peninsula such as Campeche and Mérida. They are quite interesting spanish colonial cities.