If you *really* want faeries to feel welcome, like anyone else, you have to give them what they "need." This is one of the most ancient presuppositions of faery lore - however, beware giving your faeries little clothes (bad things happen when masters make clothes for their wee ones in faery tales).
Consider: what is the purpose of this alter? Is it for you, or for "them"?
If it's the first, the purpose is to act for you as a mental reminder, a physical object to assist in turning your thoughts in the direction you wish... decorate your room/alter/etc any way you wish because it's yours.
If it's for the Fae, you need to think outside the box. If you have experience with offerings for saints, santos, or practice voodoo, you may be more familiar with this line of thinking. Basically, what does a faery need to feel comfortable?
A little home is nice for those you wish to stay, and they're not particular with real estate - making a popsicle stick house, building a little mini-tree fort outside, just a place intentionally set up for their inhabiting. Sweets, faery lore suggests milk and sweets go over well.
I've not tried liquor... I know plenty of fae who'd appreciate it, but I'm not sure I want drunk faeries hanging around... they have a strange enough sense of humor as it is.
And, this is now the realm of Alissa being completely whacked: notice what disappears around your house most often... not because the kids took it, not because you misplaced it for a few weeks... what always disappears, no matter how often you buy a replacement? For me, it's Carmex (a type of lip balm), and it's the eensy weensy oil can I use for my spinning wheel... I have no idea why I cannot keep these items in the hosue, nor why a Fae would want either but I'll be darned if those two particular items disappear with regularity, no matter how careful I am about replacing it, keeping it stored in the same spot, etc.
The faeries in my house love my craft room, the spare bedroom that my piano is in. There are lots of faery statues, faery hanging things, and pictures of fae in that room... perhaps that tells them they are welcome there. Things go missing in that room with disturbing regularity - and it's the one room in the house we never "babyproofed" so we've kept our son from playing in there all his life... I know it's not him. I also only go in there most days to do laundry and that's it (or play pianer a bit).