ferry trip is ok. head for the bar – the rest of the trip is easy.
food – monty’s
try the lounge for some drinks if you are lucky some locals in playing some music.
sights – in lerwick,not sure there is enough for a whole day! the museum, wander round the harbour, look through the few shops, clickimin broch and possibly the sands of sound/voe of sound.
http://www.shetland-museum.org.uk/
http://shetlopedia.com/Sands_O’_Sound
outside lerwick – you really need your own transport – I would say either right to the south – sumburgh head, Jarlshof and especially St ninians isle.
or head right north to Eshaness & have a wander round the cliffs.
fingers the weather is good. scenery is fabulous when the sun shines!!
good luck and hope you enjoy.
Iain