A great alternative to the busy A14 is this dirt road along the coast that passes through the Saadani NP. There are animals in the park but you will probably not see many. This is anyway not the main reason to take this route. It is much more the great track, where you can cycle without almost any other traffic along the coast through a beautiful savannah scenery and later passing many nice villages. For the park you have to pay a 30$ entrance fee, valid for 24h, enough to cycle through and stay overnight in Saadani village.
Bagamayo is currently the end of the tarred road. But in 2013 construction was underway to pave the road to Msata. The rest is a dirt road, mostly quite good, sometimes a bit bumpy.
There are cheap guesthouses in Saadani and Pangani.
In Bagamoyo, Saadani, Pangani and Muheza there are simple shops and restaurants.