Your old phone is essentially the same as a tablet. You can do anything except make calls over wifi. When you are not on wifi you can use it like in airplane mode. Here are some apps you might find useful.
– Maps.me for gps is very good but a bit quirky to get use too.
– Pocket for saving webpages for offline. Ie tide tables
– A PDF reader you may have a preference already.
– A Notes App. I like OneNote. I use this for packing & camp recipes
– Google Play Music for storing your old music
– TripIT for trip planning. I use this for my kayak trips planning.
– Google Hangouts to make calls over wifi to landlines.
– Your camera will still work to take photos.
Have fun and it will be interesting to see what people are using.