PDXGolfer, we have started our Maui vacations out more times than not with a stay in Hana for a few nights and we absolutely love it there.
I disagree that one cannot do a "partial trip". Truthfully a lot of folks are kind of disappointed when they arrive in Hana because they have heard so, so much about the infamous RTH and don't know anything about Hana itself. For most, the best part of the RTH is the drive and the incredible scenery along the way. Big caution to folks with motion sickness considering this drive!
I'm with DaveNW and also have the Gypsy Guide App (and liked it) but I found we had some trouble with it working through several stretches. There is a good audio CD that describes the route mile by mile as well and the gas station right by the McDonald's in Kahului on Dairy Rd has them. (It's where Mr Pineapple was until recently as well)
If your kids are young, just drive as far as Twin Falls, (go early to avoid a lot of traffic on RTH & to get parking at the stops along the way) find a parking place and do the easy hike to the first falls. The 2nd falls is a bit more rigorous hike so I would not try that with young kids.
The Keanae peninsula is a bit more than halfway to Hana and beautiful if your family is up to driving that far. Just remember you will need a similar amount of time to turn around and get back.
You can get some of Auntie Sandy's onolicious banana bread there and see a beautiful little historic church. Very, very pretty area.
Even if you just drive as far as Twin Falls you will get a good little taste of the RTH and see some beautiful scenery. Either on your way out or back toward Kahului, be sure to turn off for the Ho'okipa Beach overlook which has both lovely, lovely views and is a great place to watch the surfers below. This is on the cliff area overlooking the beach below. I would not try to use the turnoff to the beach parking especially if it's good surfing time as those spots are much coveted by locals and surfers. Plus, I like the view from above.
This is located just a short ways past Mama's Fish House as you drive past Paia toward Hana.
For anyone planning on driving the RTH in the near future, they have had some terrific storms that have caused a number of serious land/rock slides that have closed the road off and on. Road closures can happen anytime, but if there are torrential rains mauka (up in the mountains), flooding and landslides are a very real danger. There were a couple of motorists that were actually caught between 2 rock slides very recently. There are renewed concerns about the road's condition so please do be very aware of weather not only the day you would drive RTH but what the weather was the days before. There is a person on the Maui TA forums that is very good to give updates on such issues (hope it's okay to refer to that resource here!)
Taterhed is correct that one will likely encounter a local trying to get to work or get home that drives this road every day and just wants to get there. They are used to the road and all its nuances so they do drive faster than we sightseeing tourists. In some cases, a BUNCH faster. PLEASE show some aloha and do pull over for them at your earliest opportunity and give them a friendly wave or chaka.
Can you imagine having to commute on this road? And with thousands of sightseers daily? I live in a very, very touristy area and understand their frustration in this regard so I try hard not to be "that" visitor.
Hope this helps! Happy planning!!