Bowling is not childish at all and a great option.
bowling nor skating is childish, but if she's not into it that's fine. childish is a bad reason to though.
bowling can be really fun, but its probably better as a double-date or with close friends or something. Two people bowling can get awkward/quiet fast if nobody has anything to say/talk about
one thing that is underrated is going out to the local national park or park or hill or mountain or whatever and hike to the top for pictures. can be tiresome but if you have a few hours lined up it can be a really rewarding experience. goes even better with good close friends that both of you would enjoy spending time with, even during a """"date""""
if you both are stay at home types you could try baking stuff together, like diy pizza or mini pies or something.
inb4creampiejokeor go out shopping (if you dont mind and she likes to, or vice versa, or somehow you both like to shop) or to a local cafe for lunch, and bring some sort of word game/conversation to discuss, like a dating truth or dare kind of thing.
for any date idea, if she enjoys a certain kind of game or art talk to her about it, ask her questions and give her an opportunity to talk about it, even if she listens usually. try to be genuinely curious about anything you talk about since sincerity comes off really well.