Decorating a big hall for a 40th party?

[VanellopeVonSchweetz99]

Any good ideas, dos & don'ts please? I'm helping a friend in a couple of months and she's on a budget. Will have 2h to do it before guests come.
We've had a good look on pinterest and she loves all the hipster weddings-in-barns-type thing; light bulb strings (so expensive though?!) and chalk board signs. It's not a sit-down do but I'm thinking little tables with battery tealights in jam jars? There will be cocktails and light bites + perhaps a candy buffet. DJ in one area. She's also thinking about a loose Gatsby and/or Casino and/or Prohibition theme, are ostrich feather arrangements tacky? Too expensive? Unethical? It's a nice looking hall with beams but she's worried it's going to look bare.
Many thanks in advance.

By autumn shops will be having Christmas lights in so you'll be spoilt for choice for long strings of fairy lights. If you want to buy well in advance then Lights4Fun, as you suggested, are great.
Bunting is a great space filler and you could even make it with paper and string if you cba sewing it. Or giant, thick paper chains made from strips of old, coloured magazines look good too.
Depending on how creative you're feeling, secure thin twigs or willows into star shapes using gaffer or masking tape, then wind strips of fabric and fairy lights through them.
Whatever you do, have fun with it!

Great points, lovely ideas, can't thank you all enough!

Bunting, fairy lights, balloons, flowers/green things in vases/jam jars on the tables - shouldn't take much.

For DH's 40th I begged his family for photos of him throughout his whole life. Pics from when he was a baby right through to when we got married.
I made a huge banner which covered one whole wall and then added funny comments to each pic.
The guests had great fun reading "the wall" and DH loved it as it was so personal.

Thanks all! Met with another friend today and had a good think. Mood lighting will be crucial so we're really going for it with fairy light strings, battery tea lights and battery church candles. In the corners of the hall we're thinking those big LED lit helium balloons, they look lovely online but has anyone seen them IRL?

P.S: Yes to photo wall! B'day girl's own suggestion in fact, and some sort of 'guess the year' ice breaker.

Poundland have strings of fairy lights for gardens. Any good to you?

If you are close to a Country Baskets and anyone you know qualifies for membership it's well worth a look. Because they supply shops their wedding and Christmas stuff is in much much earlier. Christmas stuff usually goes on sale in the summer.

Will check it out, thanks!

I made yards and yards of bunting for my 60 th
I mean loads
It was great...didn’t take that long to be honest.
I bought polka dot cotton...we had a 50s theme...I picked colours that looked good front and back as I wasn’t doing them double
Cut them out with pinking shears so they didn’t need hemming and machined them onto tape
Also ordered loads of polka dot balloons which were quite cheap

Filled the space up fantastically

Also had a net with balloons in with a dozen prizes in them...ie a ticket