I do not recommend leaving valuables in the strollers, there are just too many people and a backpack might be too tempting. We had no problems taking the backpack on any ride as they all have room to put them.
ETA: We had the diapers, wipes, and clothes in a transparent bag that we left with the stroller as well as sippy cups and such. My backpack had wallets, park admission cards, cell phones, sunglasses, etc. so I consider those valuables. I had no problem leaving other stuff.
We did not lock our stroller, but it is a cheapie, and had bright colored tape all over it, so not all that appealing to a thief -and not likely to be mistakenly taken by someone with the same model. If you have a nice one, you might want to consider locking the wheels together. You aren't allowed to lock it to fences or anything, as the CMs move them when needed to make more room.