That's brilliant you and your kids are enjoying your first set. Not a bad idea to test out their interest before you invest in bigger, more expensive stuff
A 2m x 1.5m board doesn't need to be too heavy. If you go down the model railway route and build a frame out of strips of plywood (or timber) and then use a thin piece of ply on top, you should have a strong and portable table. Keeping scenery reasonably light and simple will help - as will having buildings you can remove when you're not using the layout.
On that size board, it is possible to have a small Scalextric layout that will be a lot of fun. There are some nice sets at different price points to choose from and track expansion sets to add some extra track. I'd always suggest building the table and then playing with some different layouts until you find one you're happy with. Then you can fix it down.
It may be worth having a look at Micro Scalextric too. It's half the size of standard Scalextric, so you can fit more into the same space and there's all the model railway scenery and building to use alongside. The downside is that there are fewer cars available and nowhere near the same range of track pieces and accessories to add, compared to the standard Scalextric range.