I spent a pretty bleak morning at Kernave in February 2010. The site itself was fairly interesting and larger than I thought it would be. The main point of interest are the 5 Hills that once housed forts but now just feature a set of steps to give a view over the surrounding area.
The plain between the hills and the river is also included in the inscription; it was here that the lower towns were originally constructed. There are archaeological findings here dating back 10 millennia, including sacrificial sites in the small forest. I trekked across this area to get a good view of the Hill forts. There are no actual remains to be seen at this site so don't expect any grand ruins, but It was interesting to see how the area was consistently inhabited.
The highlight for me was climbing up the Hill forts. There are no hand rails which would normally be fine but there was a fair bit of snow and ice on them when I visited so made for a somewhat precarious climb, this did make the view from the top even more rewarding though. My visit coincided with a relative thaw in temperatures (it was about 2°C but felt much colder on top of the windy hills) and as such there were streams of meltwater running along almost every path, making for some very soggy and perilous walks.
A real word of warning though for visitors, especially in winter. The Bus connections from Vilnius are not really geared for sightseeing. (Bus times can be found here). I ended up here for 3 hours and nothing was open. In fairness I knew all this in advance, a friend who visited in Summer said that there was very little to keep you entertained beyond an hour or so and the Tourist information centre in Vilnius told me not to bother making the trip, however she was very unhelpful and more interested in her own phone calls than in helping tourists so I glossed over her perhaps accurate recommendation.
It was an interesting if cold and slightly too lengthy a visit. I imagine a quick trip in Summer would be a very enjoyable and interesting experience.