At this age children really enjoy learning all about the world around them. Let them play in all different environments, like sand, dirt, mud, water, or in the woods - always keeping an aye on them, of course. A great way for water play is to provide cups and bowls and other toys during bath-time. Remember that children should never be left alone with any volume of water for any length of time.
Toddler's favourite word right now is probably "no!". This is actually a good thing, as he's becoming independent and secure enough in himself to want to try to do things for himself. The downside of this independence is increasing tantrums when he's told what to do - he may hit, bite, scream, kick, and go red in the face with sheer stubborness.