Take a boat trip along lake Nero. You will see remarkable landscapes. You will see castle - kremlin, Spaso-Jacovlev Dmirtievsky monastery and wonderful nature.
Trip lasts about 45 min and cost 50 roubles per adult (25 r per child).
Locals will offer you a motor boat trip around the small island. Worth to take for kids - high speed and a lot of fun. Though the whole voyage will take only 15 min - that is not too much for 200 roubles (4 people x 50 r).
The town is 3 hr express train ride (along the elektrichka line)from Moscow's Yaroslavsky Station. Local trains leave regularly throughout the day, but the express departs only once - at 8.30 am.
First class tickets on the express cost 200 RUR. The return express leaves Rostov at 5.18 pm every day.