I've just come across a newish teaching course run by Cambridge (the people who do the CELTA and DELTA) called the Teaching Knowledge Test or TKT, which is designed to be an entry-level English teaching qualification. It's shorter and cheaper than the CELTA and is presented as a series of modules taught face-to-face and, as far as I know, it's part-time.
I had a look at the TKT textbook to see what the course was like and was surprised and pleased to see that it covers a lot of ground, not only in terms of basic grammar, but also the practical side of planning and assessment. I took my Diploma in 1998 and have been in and out of teaching ever since, and I'm finding the book to be great refresher training! The language school I'm working at is looking at introducing the course soon, so I'll know a bit more about it by the end of the year. (No, this isn't an ad!)
I think that, on balance, the Cambridge qualifications are the most widely recognised ones in language teaching, but I do understand that the Cert is a bit of a commitment, especially at the beginning and if you have kids. I was interested to hear about i-to-i: it seems to be well-organised and supervised. Being observed in the classroom is a very important part of developing as a teacher, though, even if it's tough, and I feel that it's the best way to consolidate theoretical learning. If you can't be part of a teaching course that provides observation, though, you could get your students to give you feedback on specific skills that you would like to develop or check; about after every 20 hours of class time seems to work well. Just make sure to design your questions to them well!
Lesson planning calls - got to go. Happy teaching!