It looks like you had a good rapport with your subjects and that they had some fun, so that's really a positive.
Here are my thoughts, for what they are worth:
Many of the compositions have a lot of headroom - it looks like your using only the center focus point, so that when you're wide, you get the subject focused right in the middle and end up with alot of room above them, esp. on verticals. Some of these you've cropped in to a different ratio, so that tells me you're maybe framing your shots too wide and cropping in. They just look akward in places. I think if you consciously try to fill your viewfinder with the subjects, it will make a difference- especially when background isn't part of the composition. Also- it's hard to control what's behind them, but if you're gonna put them under a tree, I think it's better if the trunk isn't right behind their heads.
Lastly, you've got some fast glass, but it looks like you're not opening up that much. It's a style preference, but I think it would help some of these shots if you were at F2.8-3.2, or F4 on your long lens. It would separate them from the background better, which draws the attention to them. At 105mm @ F4, a shot like #7 would still retain the vibe of the place, but it would help remove some of the distracting background elements.
You have a good start, it's just a matter of shooting/practicing you can and trying different things.
Cheers,
Phil