A semi-random source of data is the block hash.

While it can be manipulated by the miner producing a block any change to the block will require looking for another block hash.  Since a solved block is worth ~$500 right now there is an economic disincentive for a miner to try and "game" the block hash.  Obviously you need to consider that risk vs the cost of an attack.  Using a block hash to protect a game where max prize is $2,000 is probably fine.  Using a block hash to determine the winner of the next power ball ($150 mil) is probably not a good idea.

Remember the higher order bytes of a block hash are not random (mostly zero to meet difficulty target) so you should use the low order ones.