I used to keep them in the living room, behind the sofa. The problem with that is they take up too much room, and are easy to accidentally knock over.
This is where the bike tree comes in. An easy (to make and use) bike stand that stores the bikes vertically to take up less room. Plus it's completely free standing - you don't need to make any holes in the wall or ceiling, and can be moved at any time (though I'd take the bikes off it first and remove the ballast). :)
Step 1: Parts
I bought all the following from a UK DIY superstore called B&Q for under 35GBP.
2x square wood decking
1x 1.8m fence post
1x bolt down fence post support
2x bike hooks (bought from Halfords)
2x stone coping (ballast)
bag of bolts for the post support
bag of screws for the decking
2x bags of sticky back felt (to use as feet)
1x electric drill and wood drill bits
good screwdriver that fits the screws you have bought
spanners or socket set for the bolts