Just get a decent airgun scope with adjustable objective. Most can focus down to within a few yards so there's no need for iron sights. I've not yet found a good system for changing back and forth since most airguns use 11mm dovetails.
There are a number of 2-7x and 3-9x airgun scopes. For the life of me I don;t understand why people go cheap on airgun scopes, particularly after spending several hundred dollars on a quality airgun.
I am partial to the Leupold 3-9x rimfire/airgun, but Beeman, Bushnell, Nikko Sterling, Hawke, RWS and several others sell airgun scopes that are popular. I recently got to try the Hawke 2-7x32 AO Map and was impressed. Dor about $125 you get AO and an external elevation knob. Optics are bright and clear. Many seem impressed with them, and I am going to give one a try.
Check out the airmax series:
http://www.hawkeoptics.com/us/riflescopes/airmax/index.php
