Wait until you arrive to book any trips, much better choice and gives you fexibility, don't gte stuck with pre-booked !
Half day trips include Elephant Village, also the pranburi temples ( about 45 minutes away ) which is very interesting, also Pala-U waterfall which gets you right into the jungle, also others to kraechan. Full day trous to brisge on the River Kwai and also around Sam Roi Yot national Park. You can also spend a day with the elephants at the santuary washing / feeding etc.
Another day trip is for snorkeling on Koh Talu island near Bang Saphan. Also the local tuk-tuks can take you for the rpice of the fare to Khao takiab temples, feed the monkeys etc.