What are you planning on doing in Yangon?

95% of my taxi drivers in Yangon spoke excellent English and were great drivers (of course, the one I got to take me back to RGN during rush hour was atrocious, kept trying to pick up more passengers-who were going elsewhere- and stopped TWICE for gas...) and could easily take you around for the day.

I would allot 1hr+ for transport into the city center, and possibly a bit more to get back to RGN because of traffic. So that leaves you at 10am-6/7pm (unsure if your flight is at 9pm, or leaving the city at 9pm).
You could hit Shwedagon, Bogyoke market, and take the circle train- no need for a guide. I enjoy the circle train/line as it shows you the local life.

I did do a food tour, Yangon Food Walks I believe, that was great- but be HUNGRY! So much food, and a nice walking tour around city hall/downtown. It was 25,000kyat, so just about $25USD.