I often go to Ban Cha-Om / Ched Khot waterfall in Nakhon Nayok province. It's about an 1 1/2 hrs ride from Bangkok. Actually, the whole mountain slope area northeast of the 33 in between the 3222 and east toward to access road to khao yai is worthwhile exploring with lots of small roads. The area around Khao sam lan national park is interesting as well.
Another option is a city ride following the Chao Phraya river up north until Bang Pa In palace/ Ayuthaya. You come across quite a few small interesting places both historical (especially around Bang Pa-In) and cultural. We've done lots of riding around the Chao Phraya and it's worthwhile following the small roads and sois and enjoy the views over the river.