Do you mean in a run or running free in your garden?
If youmean running free in your garden then you need to make sure that there are no places a bunny could dig/chew/jump his way out of. Also that no animals, i.e. cats or whatever can get into the garden. In addition to that you need to make sure that all the plants that you have in your garden are safe for your bunny, many things are toxic so if you decide to let him run free, and don't know what certain plants are, it's better to err on the side of caution and remove them.
If you intend to let him run in a run, I can't see a problem with that.My buns love their runs (we have several), and it's far safer for them.You can get fairly cheap runs that are pretty big, just make sure that they havea lid, and the bunny has plenty of shade in it. Also things like toilet rolls and flower pots make great toys.