Chances are, if you are writing App Inventor apps for Android, you already have an Android smart phone or tablet. If not, Amazon has slashed prices on their Amazon Fire Phone, 32GB (Unlocked GSM) and their Amazon Fire HD 7 tablet (but do see NOTE below!):
NOTE – the Fire HD runs Android but its not a fully open Android tablet. However, there are some work arounds; more info on that here. Here is a CNet web page that shows specifically how to load other apps – there are some limitations so read the whole link. I do not own this tablet and have not tested it with App Inventor. Appears to be the same issue with the Fire Phone – Amazon restricts it but its just a matter of setting some options in Settings to open it up. Would be interesting to hear from others if they have App Inventor apps running on this tablet or phone. Seems like they should work!
Another excellent tablet option is the ASUS Google Nexus 7 Android Tablet (16gb) which is also available in 32 GB: Nexus 7 from Google (7-Inch, 32 GB, Black) by ASUS (2013) Tablet. I have the ASUS Nexus 7 and really like it a lot.