Sounds a bit like my LO, he loves to go out and about but then misses home and when we come back it's like he is reunited with his 'long lost' toys and books, but when we have a home day he grizzles around the place like he's been locked in for months on end! We're out 5 or 6 days per week.
My DS went from unable to sit for more than about 5 seconds without wobbling over to sitting unaided (with just the odd fall) at 5.5 months, it happened almost over night, amazing how those developments appear to just suddenly happen.
You should use the next couple of weeks to read up recipes and even get a few things in the freezer in readiness. I was shocked by how much time solids took up and the first few weeks had me run ragged with cooking. Each meal took around 45 mins too which really cuts into play time. I found DS was exhausted by the end of a meal and had to go straight to a nap (lots of learning and stimulation of flavours and textures I suppose, plus the meal only just fitted in his A time), also had some disturbed nights with solids intro like Lemonthyme said.