Try using a lead...Maggie (12.5 weeks old) has been let out into the garden every hour since we brought her home at the beginning of January (we'll be taking her outside for the 1st time this weekend).
She has learnt where the "hidden treasures" are in the garden (ie rotten figs that have fallen off a tree) and constantly tries to eat them (sometimes with success). She is beginning to act up a bit whilst outside; whilst it is mostly playing and running around (including ripping up some plants and chewing twigs), she sometimes gets a bit over excited and it can take a bit of coaxing (with treats) to get her to calm down and get back inside.
My biggest concern about her in the garden is her finding and eating rotten fruit (as well as bits of her own poo that we might not have reached in time - this is something that is particularly disgusting - it's the subject of my previous post).
However, I've found a solution!!

We are now only taking her into the garden on an extending lead. This enables her to have a fair amount of freedom to run about and play but it enables us to gently pull her away from a fig she has spotted or from something rather less palatable (to us, at least)... She also seems to be slightly calmer while on the lead - although she doesn't like it being put on and is a bit perturbed at the restriction, she does seem to be calmer.
Since we'll be introducing her to the outside world properly this weekend, I also imagine that having her on the lead in the garden will get her a bit used to being on a lead and make things a bit easier on our first foray outside.
Otherwise, when she barks at 11pm or 6am whilst outside, there's not much we can really do - hopefully it doesn't bother the neighbours too much!!