blocks, wooden, lots of them.
Kids also got a lot of mileage out of dollhouse (castles) with lots of figurines. Come to think of it, I think the figurines are as valuable as the house.
OMG, this reminded me of "Super Fort":
Of course, you can *basically* replicate it with household materials and play silks (or sheets). But once kids are of crawling age, they enjoy it, up to much older children who can really build large, complicated, structures.
Blocks and stuffed animals. The way little boys play with stuffed animals is really different as they get older, but still interesting. While wooden blocks are great, don't forget that they can build big forts out of the folded cardboard ones with the brick patterns on them. Those may be practical for you if you have space to store them.
I find that my kid makes a lot of his own toys, usually out of discarded packaging he thinks is cool, and duct tape, thumbtacks, markers, pushpins, plastic film canisters.
Someone mentioned cardboard boxes. Even babies love the boxes. Blocks. Those stacking boxes have gotten almost constant use in the house for 7 years. Beanie-baby size stuffed animals. Silk scarves. Animals, all kinds of materials from wooden to Schleich-style to Playmobil chunkies. Dolls, especially the little ones in this house. Balls.
Those giant cardboard blocks that look like bricks and are very sturdy. Good ones are hard to find; we got ours from Rosie Hippo years ago, when our eldest child was two, and they are still played with once or twice a week. (Our eldest is now eight).
Other toys that have maintained their value as the kids grew older are the play silks (now used as ninja scarves and backdrops for Barbie and action figure play), TRIO building toys (from Fisher Price), Zoobs (building toys as well), and Legos.
Not as all-ages, but some of the lauri toys are really great for traveling (small puzzles, stuff that older kids could easily do with a younger kid who still needs help.).
Some bingo games with just pictures are nice, and travel well too.
Come back after the summer - I'd love to hear what works out best for you. It's an interesting dilemma.