I just wear what I always do... Skinny jeans/chinos (yes, they can be uncomfortable), a generic t-shirt/hoodie and Vans/Converse. (depending on how I feel on the day)
I'll usually take my Superdry coat for convenience of pockets/waterproofness as well, although I don't tend to take much with me to parks.
Skinny jeans.... I wouldn't go for that - I love my converse for parks though! I do tend to not take waterproof stuff but *touch wood* so far I've never had to need waterproofs for raining...