It's hard to imagine a time that Sesame Street wasn't on the air, but here's the pitch video that was made to sell it to networks back in 1968 (it was altered in 1969 once a name was decided upon). Muppet Wiki describes it as such: "The film features Rowlf describing what the new show will be like to Kermit. Interspersed between the explanatory scenes are board room sequences, in which various Muppets try to come up with a name for the show. The reel is also marked by extensive use of the word 'groovy' and phrases like 'I'm hip.'" Dig it:
Here are parts 2 and 3. Last year we visited the Sesame Street set, which has barely been altered in its 40 years.