Option 0: Solve this with hardware!
I'll start with the alternative: Buy a wireless repeater. This is just a little box that repeats the wireless signal onto the main router. Sounds just what you're after because it means you won't have to be there for things to work.
They don't even cost that much.
Option 1: Ad-Hoc networking
But failing that, you will need two wifi cards in your machine for this to work. You can buy an external one if (as I suspect) you only have one wireless card but this is why I suggested a repeater.
After that, you can either use Ad-Hoc networking by using Network Manager to create a new wireless connection. Give it a name, set some security and tell it to share the network connection... And that's about it. Ad-Hoc isn't that robust and isn't supported by all devices (my Nokia N900 used to really hate it).
Option 2: hostapd
Then there's hostapd. This should expose the network card as a full access point provided it supports the hardware. If you have to buy hardware specifically for this, you should be able to get something that they support.
It's just a bit more fuss to get going.
Option 3: another router
If your laptop also has a ethernet card, you could use Network Manager to share your connection over it and route that into the WAN port of another router. This might be viable if you have a router lying around and you don't mind your laptop being tied to it.
Just edit the
Auto eth0 connection so that under IPv4 Settings, the method is "Shared to other computers" and that should be about it. Similarly if both computers can be joined together, you could just use direct network cable with this setting.