Big Kid Party! :)

If you want it to be nice, I would do it a "Barefoot Contessa" way.
She will take white table cloths, nice silverware, a few clear, tall vases filled with freesia or small vases w/white tulips.
White plates.
White cloth napkins make things have a nicer feel.
Don't use cheese decorations like streamers.
You coul hang a few (5) colorful balls. They have these that look like
pom-poms so they aren't cheesy looking.
String up white lights around the backyard for a bistro vibe.
I wouldn't do any games or activities. You could have a fancy notebook
that guests could take a page out of, write down a funny thing about the
guest or a wish for her 30th year then have them drop into a fancy silver
bowl.
I wouldn't do any activities. I would just have music playing, candles in
lanterns (so they can't get tipped over).
A toast to the brithday girl.
Maybe some old pictures of her up if she won't get upset.

Edit: I would add those little light stakes around your backyard for cute ambience. You can get them at the Dollar Store.

I would just do some little bunches of flowers and plants spaced around outside. You can use short glasses and cut off the flowers accordingly. They can be the same or mixed matched. It might be fun to get some colorful plastic table clothes or even the red/white checked ones too!

As far as activities, I would just get some for the kids to do. Maybe some western theme stuff if you're BBQing or some summer type stuff. Have a craft table with tattoos, etc.

I like to fill large Rubbermaid type tubs with water and Joy liquid soap (at least a whole bottle/tub usually) and just throw a bunch of stuff in there for bubbles. It's self serve for the kids and it's big enough that everyone can share and it won't get spilled! Kids might get wet and bubbly, but that's better than someone crying over spilled bubble containers! I use hangers, spatulas with holes, string tied together, cut off soda bottles, yogurt containers, etc. Basically anything with a hole works!

You can also have sidewalk chalk available and maybe some balloons and other toys around.

Black streamers and black balloons. A big banner that states '30 isn't old - if you're a tree'. Have fun.

Look on Oriental Trading. We went to a surprise party for a friend and I purchased 30th Birthday party coolie cups (koozies) and gave everyone one when they walked in the door!

I know I sound mean, but for a 30th birthday, I would have adults only! Go to a big party store or on line (Oriental Trading) for decorations. I personally would do something pretty, lots of flowers etc. pretty bright colors. Trader Joes has very inexpensive beautiful flowers. Bright and cheerful.