If she doesn't want a bike of her own then an xtracycle conversion or tandem is your best bet.

With an xtracycle, you could still use it as a utility bike in case you broke up with her (hey, it happens...)

A tandem would most likely be more romantic in her mind, which might bring it's own set of rewards.

Either one is going to take some extra room for storage due to the extra length of the bike. Neither one is going to be efficient for you as a commuter (since you highlighted that aspect in your thread) unless you haul a bunch of freight, in which case the xtracycle would work. A folding tandem might help in terms of storage space, but then again you might be stuck with a tandem. She might stick around but then changer her mind and not like bikes anymore (it happens).

Short answer-there's no cheap option other than the pegs. Give her the peg treatment for a while and if she sticks with it and stays interested, then maybe it will warrant the additional cost of a tandem, xtracycle conversion, or a bike of her own. I'm not too experienced with pegs myself but if you're using the quick release clamps they might not be compatible.