I was on Tinian in 2010 - awesome trip!! The only thing that comes to mind is that you cannot drink unbottled water on Saipan. The hotels even have big placards by the sinks that say not to drink the water. We stayed at a place near the airport - it was low key and nice. I know there are much nicer places on the North end but for the $$ this worked great.
You can see several of the Japanese bunkers and so forth on the airport but Iam notsure how easy it is to actually go visit them - the airport is still an active international airport with TSA and all

I'll see if I still have the tour guide service # we used on Saipan. The guys name was Richard and he was great. Be sure to hit the Hard Rock Cafe!
North Field on Tinian however is a whole different story. You can rent a moped or car and spend weeks roaming through the jungle where the "busiest airport in the world" once was. There are several monuments, you can visit the old ruined Japanese facilities, the atomic bomb pits and the foundations of the bomb assembly buildings. I recommend trying to geta hold of Don Farrell on Tinian - he is the resident expert on the history of the island (as well as most of the Marianas). I stayed at the Tinian Dynasty Hotel/Casino and it was nice - kind of like finding Ceaser's Palace on an island - and mostly empty at that! The other one I can recommend is Flemings Motel - it is a great little place for a good price and they have an awesome cafe attached that is AWESOME. If you go, tell Deb Fleming "hi" from me.
Tom P.