I have been to all 3 places- Sesame Place, Hershey, and Dutch Wonderland many times with my kids (ages 7,5 & 22 months) and I think Dutch Wonderland is the best for a 15 month old. We love amusement parks, and have been to all 3 with our kids at around that age- Dutch Wonderland is definately the best for that age. Our first visit was when my son was about that age, and I was pregnant with our second.... and we were able to do most everything. Ever since then we've gone at least twice a summer. A big reason why we love it, besides the fact that little one can do so much, is that the lines are hardly ever long for the rides. Even if the park looks crowded, the waits are not long at all. There have only ever been 2 times when the lines have been horrible, and one was when we visited during Labor Day weekend. (and horrible to us is having to wait more than 5-10 mins!)
There is tons to do around the lancaster area as well- try eating at Miller's smorgasboard. It's pricey but really good. It's a great place for a long weekend.