Haven't read all of the replies, just wanted to chime in. For my DD's 10th birthday we did a small "spa day" party here at home (just DD and two friends). They arrived in time for a "fancy" spa-style luncheon on the deck (served by a waitress who looked suspiciously like Mom

). It was a while ago but as I recall it was fruit salad, fancy lemonade in margarita glasses that were rimmed in pink sugar, crustless sandwiches, etc. Cake was a pink-frosted small three tiered cake covered in fresh daylilies (again, made by a baker who looked a lot like Mom).
The events of the day included mani-pedi's (again... Mom

).... I already owned a foot spa and borrowed a second from a friend. I laid out a table full of spa supplies (foot wash, foot/hand scrub, files, polish, towels) and was prepared to do the work myself, but the girls actually opted to take turns with each other, and a swim in our pool followed by a soak in our hot tub (yes, I realize everyone wouldn't have that available at home). Next up was gift opening and mock-sleepover with a movie in our playroom (hung dark paper over the windows to make it seems like a theater). Popcorn and pizza for dinner during the movie and (and they put on their jammies) and then HOME with their parents! SUPER easy and affordable!
A few of the $$ saving things I did....
1. only TWO friends... keeps it really simple and affordable.
2. did my own baking/cooking (I hate both, but kept is easier.)
3. my own decor (bought a beautiful remnant piece of pink paisley fabric. Cut it to size for a tablecloth, used the extra piece to make napkin rings), bought plain pink plates and napkins, used real flatware and bought 4 plastic margarita-style glasses, decorated the cake and the table with flowers from my own yard plus pink and white balloons (I did splurge on a helium tank from the party store). By FAR one of my cheapest parties, but the girls had a blast.
4. MOCK-sleepover... no worries about them staying up all night and no need to feed them breakfast. We had had problems with crying kids during sleepovers in the past (and the need for parents to come out at 1am to retrieve said-crying child), so no more sleepovers for a while.
5. Goodie bags were super easy to theme.... nail polish, files, lotion, etc! Used coupons and such to buy nicer things, but fewer of them.
The decor:
Hidden under those huge flowers was a pretty pink cake!
HTH..............P