As others have said, the Apple II is and always was a hobbyist system. In fact, the Apple II series used to come with a little booklet entitled "A Touch of AppleSoft BASIC" which explained how to write programs for the computer.
Some of its treasures do lie in commercial software. I feel the "must haves" are the original Print Shop, the old version of Oregon Trail (you haven't lived until you've controlled a little man with a rifle using arrow keys during hunting sessions), Fantavision, and, if you can find it, AppleWriter II by Paul Lutus. VisiCalc is also nice to have for its historical significance (I don't have a copy of it so I can't vouch much for it though). If you're into games, Lode Runner is the essential classic (again, I don't own this one and haven't played it since 1992 on a friend's IIGS).
Part of the fun of the Apple II is the card scheme. Macs aren't known for expansion cards (unless you're into the II/IIx/IIfx with six NuBus slots) yet the II, II+, and IIe require them to do much of anything. Later models came with disk drives and ports built in and also included printer ports instead of Super Serial Cards.
Of the ports that are included on the early Apple IIs there is a cassette input/output port (if 5.25" disks feel too modern for you, try using tapes!), a joystick port, and a composite video port (if you don't get an Apple II monitor you can just plug it into your TV).
If you're just starting out with an Apple II I recommend a IIc because of its ports. You won't have to mess with cards yet can get used to the feel of the interface, the style of the programs, and ProDOS. The IIc is also easy to plug into a TV since it's small, has a disk drive built in, and can sit comfortably on your lap as you work in front of a TV. (I recommend a monitor if you can find a good one though).
Once you've gotten used to the IIc I suggest a IIe. This is THE essential Apple II machine, complete with a maddening array of cards. Go for Disk II drives if you can find them for a truly retro feel.
The machines are fairly easy to find on eBay and so is the software, often sold by the bushel. Make sure you buy genuine, legal Apple II software (pirated disks were very common during the Apple II days and still tend to show up on eBay). If you want to use an ImageWriter II, make sure you get the right cable for it (the Super Serial Card uses one connector, the IIc another, and the IIc+/IIGS use the same as the Mac Plus and above).
If you don't want to go on eBay you can check local school surplus and yard sales, but be warned--these machines aren't as commonly found in surplus as they were ten years ago. Some recyclers don't even refurb them anymore and just send them right to the compactor.
I think you'll find working on an Apple II to be quite easy and very enjoyable. My first computing experience came on an Apple IIe on the afternoon of April 16, 1989 and it got me hooked right away.