Danube Delta. We drove up from Constanta and followed the coast around Lake Razim as far as Murighiol (both of which are inside the WHS boundary) and then on to Tulcea the main centre. As I said in the review this isn't really "seeing" the Delta, for which you need to take a ferry to places away from the road and invest several days but it will give you a general feel and a chance to see some of the birding spots.
Villages with fortified Churches. Most of these villages are scattered NNE of Sibiu along/off route 14 between Copsa Mica and Sighisoara although Viscri is SW of Sighisoara towards Brasov. When we were there in the early 90s it was all wonderfully rural along gravel roads. The "Saxons" were getting out to Germany and the area seemed in decline - some of the churches were in bad condition and the only one we went inside was Biertan. When the site was originally conceived this was in fact the sole nominated church and the site was extended later. It is massively impressive and, if you can only take in 1 that is the 1 to choose. It is about 9kms off the main road east of Medias. However the Saxon villages are very different from those in other parts of Romania and half the fun would be to search out several villages when you get there
Churches of Moldavia. My vote for the "highlight" WHS. In NE near Ukraine/Moldova -Vatra Dornei is the nearest "town" and quite pleasant - was a "resort" in Habsburg days. The scenery as well as the Monasteries is very fine and even at my fast travelling speed I would give the area 1.5/2 days. You need to make a circuit to take in a number of the monasteries - we did approx Campulung Moldovenesc , Gura Humorului, Marginea, Vatra Moldovita, and back to Vatra Dornei. We took in around 5/6 monasteries - in particular Voronet, Humorului, Sucevita and Moldovija. There were even then lots of "log cabin" hotels (it is a major logging area) B+Bs etc.
Monastery of Horezu. In southern Romania Between Pitesti and Targu Jiu. I found it a bit "tame" in comparison to the Moldavian churches. I also REALLY regret not going on to Targu Jiu but we just didn't have enough time. As a result I missed the Brancusi "Endless Column" - a "sculpture" of world significance AND on the TList now dammit!!
Sighisoara. Well you can't miss it - you will go to it before/after the Fortified Churches.
Our general route in 8 days was Bucharest, Constanta, Tulcea, Brasov, Bucovina/Moldavia, Transylvania, Sibiu and back to Bucharest. with lots of minor diversions