my kids & I started a new @LEGO_Group project last week: Tower Bridge (#10214, 4,295 pieces). The pieces don't come in numbered bags, so it's great for 3 people. Two kids each build a tower, & the parent searches for pieces. I spent the first day just creating a sorting system...

If u have 4 Lego enthusiasts, I suggest Taj Mahal (#10256, 5,923 pieces) bcuz of the hi degree of symmetry. Most elements are constructed 4x, so your kids can each build one & compare. Teaches how parallel process mgmt can be used as a form of engineering quality control ...

If you're looking for a big Lego that's straightforward, brightly coloured, and more of a casual project, try London Bus (#10258, 1,686 pieces). It's nominally marked "Creator Expert" but it's fairly simple. The only real chance to screw up is with the stickers. So be careful ...

By contrast, Saturn V rocket is trickier (#21309, 1,969 pieces). a good project for a teenage kid; or a young kid paired with an attentive adult. A single early hidden error in core construction can require latter teardown & restart. Which itself is a great engineering lesson...

(the error in our house resulted from my 7 yr old skipping placement of a single round 1-cell minibrick, creating a radial gap that thwarted a claw-&-pole assembly much later. I analogized this to the real O-ring that caused shuttle tragedy of '86, tho my kid wasnt interested)...

The last Lego model whose construction I can describe firsthand is Statue of Liberty (#21042, 1,685 pieces). As with, Saturn V, any early error can compromise latter integrity. This was the 1st of the 5 Legos we've done in COVID era. Internal tensile base-statue connector is cool

I don't know how long isolation will last, but projects we have our eye on in are: millenium falcon (#75192, 7,500 pcs... Watch out, there are multiple Lego MFs), R2D2, Maersk train, & big Maersk cargo ship (again, watch out. There are 2 Maersk ships) https://www.lego.com/en-ca/product/maersk-line-triple-e-10241

These are expensive
But u can get ~half your $ back if u sell them later online.(Some ppl buy them constructed. others buy them constructed, then disassemble/reassemble, or ask u to disassemble.)
Also, cost is less than Σ {all the other activities we've canceled over last month}

by the way, if you're looking for a cheap way to get into the hobby, I find that on Kijiji, you can sometimes find people getting rid of huge bins of Lego for fairly cheap. My kids have always liked to build freeform projects, in addition to the programmed instruction sets....

even if you only do the programmed instruction sets, it's always useful to have these big bins around, as they typically will provide backup in case you are missing one or two pieces