If it's your first time in Thailand, I think you can follow the general guide book. Beware of the scammer near the Grand Palace. They will say the Grand Palace is closed for a ceremony and offer to take you to other places. The Grand Palace is "open every day" from 8.30 am - 3.30 pm "even with a ceremony"!!! Don't take Tuk-Tuk, take a "metering taxi" only. Don't comply with any taxi which doesn't turn on the meter. I would recommend a boat trip to see BKK along the river, the ticket is very very cheap too. It's worth the money. Besides, I would suggest visiting a small floating market around BKK or spend one or two nights at Amphawa/ Samut Songkram. ( 14 floating markets around BKK https://travel.trueid.net/detail/mDrV1EzLwx5)
I have no info about BL or where the BL fan goes :-) But if you have a favorite actor and you want to see him face-to-face, you can check out his schedule from the FC and ask his FC to help you. It's quite easy to meet your favorite stars in Thailand. You can take a photo with them too if they don't have so tight schedule. Especially, when you tell his FC that you're from aboard, the FC will surely get you to have a photo with him even with his tight schedule. Thais are pretty friendly.
Food: Yes, try a variety of foods as much as you can. Street foods have good quality and cheap (and of course, tasty!)
Chiang Mai: Unique culture with history, nature -- good for hiking, visiting elephant camps and good foods. You might want to diverse your trip to Chang-rai to see "white temple" and "black house" if you have time.
Lastly, if you are a woman and go there alone, don't stay out late at night and beware of a pickpocket.
Have a good trip!
