A looong time ago I read something that struck me as odd, but it turned out to be true: You'll get the most value from your dollar from the really expensive toys ... and the dollar-store toys. It's the in-between things that get little play time.
The trampoline was the single item that received the most playtime of anything my kids had. Obviously they jumped on it, but they'd also go out and lay on it to read. A couple times they slept out on it. Other things that they loved intensely for years: Their playhouse, their sandbox, their huge doll house, their American Girls dolls. Oh, and their table/chair set and play kitchen -- those were used constantly.
And the little dollar-store type stuff was loved fiercely for a couple days, then discarded. For example, a little tea set or a yoyo. Still, for a dollar, it wasn't bad.
Something else I read once in a John Rosemond book (Christian author who writes about parenting): Allow your kid to have no more than ten toys at a time. If the child has too many toys, he plays with none and develops no imagination. If he has only a few, they get used. If my children were little again, I'd be more strict about this.
Other thoughts:
- Don't skimp on art supplies
- An outdoor item that hasn't been mentioned: Tether ball -- bonus: you can play alone
- Kids need to own books, but that's no substitute for the library
- Buy plenty of board games and play them regularly as a family
- When you buy a Christmas present, etc., consider going with something that can become a collection and be "added to" later: For example, Lego kids always want more ... buy the American Girl doll for Christmas this year, and instead of another doll next year, add clothes or accessories ... dress up boxes need to be replenished with more stuff ...
On the subject of dress-up boxes: That was another top toy -- my kids used it daily. Last Christmas one of my no-children male colleagues was asking what he should get his 3-year old niece for a gift. One of us ladies said, "A big tub with a few dress up items, then keep adding to it for years". He was very uncertain, saying that didn't sound fun at all, and how long would that hold a girl's attention anyway? All of us with girls assured him it was the perfect gift, and our teens still secretly enjoyed such things. He bought it and said he was named Uncle of the Year for that gift.
You mentioned a park nearby. Whatever it DOESN'T have, that's what you should get in your own yard. If that park has great swings, I wouldn't bother with them in your own yard -- I'd go with a good sandbox instead. If the park doesn't have a slide, get a Little Tikes model slide. (another big favorite at my house: The $2 yard sale slide -- as toddlers, my kids would slide 50Xs in a row, giggling all the while ... and in the summer we'd put it over the edge of their kiddie pool -- that was even better)
Someone mentioned a toy library. We were members of a toy library when my kids were small, and WOW was it great. We could check out "tubs" full of themed-items. Perhaps this week we'd check out a box of puppets, and two weeks later we'd get a science experiment tub, then the next week it might be a doll house. It was out of our way, but it was worth the drive.
Final thought: Don't feel that you have to entertain your kids. I'm not saying ignore them, but by-and-large, we Americans are raising a generation that doesn't know how to entertain themselves or simply be quiet with their thoughts for a moment.