There are literally loads of places to visit in Northumberland. Lindisfarne also has a castle worth visiting.
Places worth visiting: Alnwick plus castle and garden Farne Islands Bamburgh castle Morpeth Chillingham Hauxley nature reserve Warkworth Belsay Hall Kielder Rothbury Cragside Woodhorn colliery museum Ford and Etal Hatherslaw light railway Ingram Valley Alnmouth Hadrian's Wall Hexham Most of the coast Wallington
I would suggest you stay in the Rothbury or Alnwick area as it is pretty central to where most of the places worth visiting are.
Make sure you check the tide timetable before visiting Holy Island. Every year there are a few idiots who get themselves on the news because they tried to cross the causeway at the wrong time and got stranded.
Not a whole lot to do on lindisfarne, there is the priory and beach etc but you could do it in a day. Alnwick has the castle and gardens, large book shop, little museum etc and is about 30 mins from lindisfarne.
Add alnmouth, seahouses, bamburgh, warkworth to the list too all not far from each other and worth a visit.
I would say staying near to alnwick would be a perfict base as near shops etc and central to most places imo.
I have a vague idea that I might quite like to visit Lindisfarne and Alnwick (are they near each other) and that area generally this summer while heavily pregnant but I don't know. What is there to actually do at Lindisfarne? Is there anything else good in the area?
We normally do a lot of very physical stuff on holiday so what can I do at about 7 or 8 mnths pregnant?
Also, we will have our new campervan by then, where could we stay with it?