Stack a tower of blocks and show your baby what happens when you knock them down. If you stack them up again, he will quickly learn to destroy your tower. For even more fun, stack the blocks out of your baby's reach. If he can crawl or otherwise reach the intended destination, he will amaze you with his speed. If he is not yet that mobile, this is certainly motivation for him to become so.
As you have noticed during bath time, babies this age love to play in water. Although this can be very enjoyable, never allow your child to be unsupervised. Also, be sure to empty all water from the bathtub, pail or any other container immediately after use. Never leave a baby alone near a bathtub or any other receptacle full of water, even for a moment. Keep the bathroom door closed; drowning can occur very quickly in even a few inches of water.