The best thing would be to enhance them with
microfons, but then you have the neighbour-problem..
This is what you could do:
Ask the neighbours if you could have their permission to
make alot of noise for an hour or so.
Ask everyone in a half mile radius.
It may seem like too much work for a small garden concert,
but the last thing you want is some cranky neighbour
calling the police in the middle of your concert.
Actually every instrument tends to sound a bit hollow
outside. Crank up the bass on yer amp!
If its wet make SURE no electric components touch the ground.
That could really ruin the whole thing.
Otherwise just do the things you do on an indoor concert.
Like writing down amp settings if some kid should
sabotage your set.
One more tip:
Hire the bassist!
Bass is really important even on an indoor concert.
But especially outdoor when everything else sounds
kind of edgy, the bass is needed.
Otherwise good luck with the concert!
Playing outside is always fun, and Im sure
this wont be an exception.