1) Bring a picnic to the park or beach.
2) Cook for her.
3) Go canoeing on a local lake (cheap here in MN -- not so sure about the UK
)
4) If you're very adventurous (or it's a longer-lasting relationship) take a toddler to the playground (niece or nephew or friend's kid if you don't have one of your own.) Perform well here and you'll get LOTS of points BTW
5) Play tennis at a local park (free here, again not sure about UK)