Photo Grid is a free (ad supported) app with lots of options for creating collages, as well as video slide shows accompanied by music.
You can select the aspect ratio for the photos, and choose from more than 300 layouts. The layouts that appear depend upon how many photos you have. You can also select background colors, frames, text, and stickers. Your finished collage can be shared with the various social networks, emailed, or saved to your camera roll.
I have used the app on several of my pictures, and find it straightforward. You point the app to your photo roll, select the photos you want to use, and the rest is pretty obvious, with controls along the bottom of the screen. There is also a feature that lets you create videos with photos. There's isn't a lot of flexibility there. Once your photos are selected, you can't change the order. And there are no dissolves between photos, which I think is a requirement for a nice looking slide show. The photos just cut from one to another. You can add music easily, and the app takes care of the timings and conforms the music to your slide show duration.