Hi there and
to the forums!
I'm glad you're looking for options to make your budgies even happier than they already are
If by "aviary" you mean "very large cage", then yes, that would be a good idea
It is not advisable to house budgies outdoors (even if you have great climate outside) unless you have taken a number of precautions (building a roof over them, fencing/netting around the enclosure to keep out predators, etc.) and it is not preferable to house them outside, so don't worry about that.
If you are financially able to invest in a nice large cage for them (a favourite around here is the Prevue Hendryx F040 or F050), that would be a great option! They will love the extra space to fly around in and explore
Here's a link with some more information:
Adding more bird is not always a good option. Budgies can only be housed with budgies in a cage, as other birds have larger beaks and can be dangerous to the tiny budgies if they are ever annoyed and lash out, even accidentally. Additionally, even adding budgies isn't a great idea if your two already get along; more birds could upset the flock dynamics and may end up needing to be housed separately anyways.
Meanwhile, ou've come to the best possible place to learn even more about budgies!
To start, be sure to read through the forum's many Budgie Articles and "stickies" (threads "stuck" to the top of each subform for easy reference) to ensure you're up to date on all the best practices for budgie care!
If you have any questions after reading through everything, be sure to ask as we'd love to help
We look forward to seeing you and your budgies around the forums!