exactly my plan! was thinking of going to sukothai OR lopuri (see sunflower fields) OR ayuthaya OR kanchanaburi OR maeklong railway market...
but we're already going to grand palace/wat pho so i'm thinking it might be temple overkill if we do sukothai/ayuthaya... then for lopuri it's the wrong season to go, we'll not see any sunflowers.. for kanchanaburi i heard it's quite underwhelming... Maeklong railway market i'm ultra keen because both my bf and i are photog buffs but if we go on our own it'll easily day an entire day, whilst if we book a private tour, it'd cost a bomb, and include other stuff (alligator farm etc) we're not so keen on... so we're still deliberating.
Ko Kred i heard it's ok but not amazing.... how you find it?
Amphawa is for the floating market right?... hmmmm
Sunflower first, I was really puzzled when I see the following at a temple on my way to Udon Thani, so hot and no rain water and yet it can grow
Places like ayutthaya, mae klong, kanchanaburi are those that near to BKK and suitable to shun bian go on a day trip. Sukhothai and Chiang Mai are way too far away. I myself planning to go to these two when got the time. I simply love the night market of CM and also visiting temple ruins. Oh ya Ayutthaya is similiar to Sukhothai, more of temples ruins instead of visiting temples like wat pho, wat arun. Lop Buri I didnt read up much, but heard is go there see monkeys

LOL
If you have an extra day when visiting BKK, can go mae klong railway market and then proceed to amphawa floating market, suggest to stay a night there. Mae Klong just about 1 and half hour by van from Victory Monument (70 bahts) and from Mae klong to Amphawa is only about 20 mins by Song Thaew costing 8 bahts each.
Sorry for the blur pictures
As for Ko Kret, I quite like that place as it is totally different from the busy district of BKK. More like Pulau Ubin, you can rent a bicycle round the island, or by foot walk the lane with stalls and shops selling unique souvenir and food.