We just returned from San Ignacio. There are several tour operators offering trips to Tikal. We stayed at Crystal Paradise and took a full day tour to Tikal the day after we arrived with no prior arrangements. I think it was $85. Their website is www.crystalparadise.com
Also try Maya Walk Adventures. www.mayawalk.com
Their tours are a few dollars cheaper, I believe. Both are quite good.
We also took a tour through Crystal Paradise to a ruin called Caracol. We went on a Monday and we were the only group there. Imagine a huge ruin with only 10 people there. The reason being, I believe, is it is a 3 hour drive (almost) down a rutted dirt road. But it was well worth it. The drive in itself was an adventure. We almost liked Caracol better than Tikal and our tour guide was very knowledgeable as he had lived and worked on the site in the early 90's. They will be paving the road soon so it won't be long until the world arrives at Caracol's doorstep.