How quickly do they need the money?
Im on DD's PTA, DS' PTA and work in a different school so these are ideas from across 3 schools (I wouldn't recommend doing all of them at the same school in the same term, the parents would hate you)
Film night. we go to the cash and carry and do a little tuck shop (which makes next to no profit but means we don't have to pay for snacks) and use a DVD one of us already owns, we do it over two evenings with different films, so one for reception and ks1 and one for ks2 and charge £5 a head.
Disco, can be put on fairly cheaply and always raises a fair amount. You will be surprised how many people know someone who can help do it very cheaply if you ask around. We then buy sweets from cash and carry and glowsticks from amazon that the children can buy.
Parents quiz night, bring your own bottle so the only real cost is the prizes. At the quiz night there will usually be a raffle and a donation pot too
Christmas jumper day, pay £2 to wear a Christmas jumper with uniform.
Make the teacher an elf, I'm not entirely sure how this came about but year 6 were told if they raised £250 in a week their teacher would dress as an elf for the entire day, they raised nearly £500 
Set the older children a challenge of who can come up with a way to raise the most money, you'll be amazed what competitive kids can squeeze out of people. We also gave groups of year 6 £10 a group to set up a stall at the Christmas fair, which their teacher linked into a maths project, they came up with some fantastic games that the younger kids went mad for
There are lots of different things you can get sponsored for doing.
Family camp, obviously one for in the warmer months but £10 per family to camp on the school field, bring your own food and drink, or buy some from the BBQ, if you can get one have a raised little fire pit and sell some marshmallows for the kids to roast. have a little tuck shop.
Then there's the Easter egg hunt, Summer and Christmas fairs.
There are companies that will buy bags of old clothes, toys etc and plenty of parents who don't mind having a clear out.
My school hire out the hall and outside areas, we get a lot in rent from various clubs that use the school in the evenings without really costing us much (as the school is still open for teachers working in their classrooms so the heating and lights are on anyway) but obviously that's not a quick fundraiser