Help me with my itinerary please

I arrive at Cancun at 1 pm.

1. is there a bus from Cancun airport or bus station into Belize? and approximate cost? and duration of the ride?

The bus company, ADO, runs a bus between Cancun City and Belize City. It stops at Cancun airport on the BC to Cancun run, but NOT on the Cancun to BC run, so you will have to get a bus from the airport to either Cancun City bus terminal or Playa del Carmen Alterna bus terminal. Playa del Carmen has two bus terminals, so if you go to the 5th Avenue terminal in Playa, you'll have to change to the bus terminal at 12th & 20th to get the bus to Belize. To further confuse you, sometimes the airport bus will stop at both bus terminals. Here are the details...

In September 2011, the Mexican bus company ADO began two new daily services to Belize City – one from Cancún and one from Mérida. The ADO buses are first class with 44 comfortable seats, video, toilet and air conditioning that seems to have only two settings, off and freeze, so dress warmly. In Belize City, buses use the Belize City bus terminal (commonly known as Novelo’s) located on West Collet Canal. These buses do not go to Chetumal in either direction.

On the Cancún route, stops include Playa del Carmen, Tulum, Corozal and Orange Walk, but passengers are not allowed to use these buses to travel within Belize, for example, to travel from Belize City to Corozal. Passengers can board the bus at Orange Walk or Corozal only if their final destinations are in Mexico. On the Belize City to Cancún run, the bus stops at Cancún airport, but on the Cancún to Belize City run, there is no stop at Cancún airport.

Cancún to Belize City
Departs from Cancún bus terminal daily at 10:15 pm with stops at Playa del Carmen (11:40 pm), Tulum (12:45 am), Corozal and Orange Walk. Arrives in Belize City about 6:30 am.
Fares to BC (Jan 2013): MX$496 from Cancún; MX$460 from Playa; MX$400 from Tulum.
Trip time: 8:10 hours from Cancún.

Belize City to Cancún
Departs from BC bus terminal daily at 7:30 pm with stops at Orange Walk (9:00 pm), Corozal (10:00 pm), Tulum (4:00 am), Playa del Carmen (5:00 am) and Cancún Airport (6:30 am). Arrives at Cancún bus terminal at 7:00 am.
Fares (2012): BZ$55.00 to Tulum; BZ$63.00 to Playa del Carmen; BZ$79.00 to Cancún.

Information & ticket sales in Belize:
Belize City: 636-5256, 669-9683
Orange Walk: 624-8200, 302-2240
Corozal: the MCD Art Shop (opposite bus terminal) opens at 8:30 pm to sell tickets; tel: 663-3014.
If dialing from outside Belize, the country code for Belize is 501.

1. I have 8-9 days. I want to do a few days by a beach, and a few days to explore the ruins, for the beach, Caye Culner or Ambergis? looking for a relax, backpackers beach scene

Caye Caulker is smaller, quieter, more laid-back, and more popular with backpackers. There are a few hostels on the caye.

1. for the ruins, if Im only confining my vacation in Belize, (saving Guatemala for another trip) is Ignacio still a good base? and which Belize Ruins are worth seeing?

San Ignacio is a good base because of the good selection of accommodation, including hostels, for all budgets. The best site in the region is Caracol. Xunantunich and Cahal Pech are smaller but more easily accessible -- no tour guides/companies necessary. Another excellent site worth seeing is Lamanai, but it's not easily accessible from San Ignacio. Maybe you could see it while travelling between CC and San Ignacio. It's a full day trip that includes a trip along a river to get to the site.

1. aside from beach and ruins, are there any spectacular, not miss, experience or place Ive missed?

Maybe the ATM cave or another cave trip. Some people want go to the Blue Hole.

As long as chatumel and Belize city is not dangerous after dark. I would prefer to boot it all the way to Belize city, spend the night there and next morning, to San Ignacio. That's also provided that I could change currency across the border at around 7pm ish.
Better yet, if there's a direct bus from the border to San Ignacio.
Do you think it's feasible and advisable to do so?

As long as chatumel and Belize city is not dangerous after dark. I would prefer to boot it all the way...

Did you mean Belize City or Cancun?? I'd say Cancun is reasonably safe at night. I've always found Chetumal to be very safe at night. Belize City is a place you really don't want to be at night. I wouldn't even suggest staying overnight there.

If you land at Cancun around 1pm, go immediately to the bus terminal in Cancun and buy your ticket. Then store your luggage and do something until the bus leaves at night. Actually, my preference would be to get the airport bus to Playa del Carmen (5th Avenue bus terminal), buy the bus ticket to Belize (some staff speak English), store my luggage at the bus terminal (storage area is upstairs; or carry your luggage with you) and then do something in Playa (better place to be than Cancun). Playa is very safe at night. You might even decide to stay there for a night, but decide before you buy the bus ticket.

The ADO bus arrives at BC around 6 to 7 in the morning, so you have plenty of time to get a water taxi to Caye Caulker or a bus to San Ignacio (now I'm confused about your plans?). Nothing to see or do in Belize City.

I've only crossed the Belize?Mexico border during the day and there have always been money changers around the Belize Immigration building. I don't know if they will be there at 3 or 4 o'clock in the morning. If you have a plastic card, go to an ATM in Belize City, Caye Caulker or San Ignacio.

There is a direct bus from Corozal to San Ignacio, but it leaves Corozal at 3 or 3:30 in the morning. I don't think the ADO bus will arrive early enough for you to get the bus to San Ignacio. I thought you were going to Caye Caulker first?? If you arrive at Corozal before 3am (unlikely) you could get off and get the bus (old school bus) to San Ignacio. My preference would be to stay on the ADO bus (more comfortable) all the way to Belize City. The ADO bus arrives at the same bus terminal where you change to a bus to San Ignacio.

That's also provided that I could change currency across the border at around 7pm ish.

Which border? Mexico/Belize or Belize/Guatemala?
In either case, money changers should still be hanging around at 7pm.

First off, you guys are AAAAmazing! thank you.
I will follow your suggestion to go to go to playa after landing in Cancun, buy my over night bus ticket to Belize city.
Arrive BC in the morning then continuing on to San Ignacio right away.
I decided that I prefer the jungle and the ruins over the beaches, that's why San Ignacio First, I will follow your recommended sites and stay 3-4 days. then to Caulker for 3 days of chill time. then take the same bus route back to Cancun airport
some more questions please
1. is there a toilet in the bus?
2. is it the same bus terminal in Belize city for me to trasfer?
3. is it fairly easy to walk around San Ignacio and Culkner and find budget accomodation or should I prebook?
4. on the way back from Caulker to BC, then To Cuncun Airport; I would prefer to take the overnight bus, arriving at Cancun airport at 7am and take my flight a few hours later at 1240PM. are the bus reliable and prompt or am I taking a major risk and should overnight at Playa the night before my flight?

Thank you again :)