I bought a travel darts board with magnetic darts from John Lewis - rolls up around the darts container.
My gtrandkids love a treasure hunt (if you have some time to devise clues!) The older ones could have slightly more challenging clues than the younger ones. A paddling pool with fountains (lots of splashy fun if it's really hot). Pebble painting (just need some acrylic paints), Building a home for an imaginary fairy or elf (usually best in a tree root if you have one)- but using only naturally materials foraged in the garden and not allowed to use anything that has to be picked or broken from plants etc. Or how about "The sock game" (2 long socks, each filled with 20-30 identical small objects (coin, pencil, dice, lego brick ... you get the picture), List the contents on a piece of paper or card and against each item, assign a playing card - eg jack of hearts = dice, 2 of clubs = 50p coin and so on. Divide players into 2 teams then shuffle the cards and turn them over, calling out the item to find - first to find it and pull it out of the sock gets a point for their team! (Actually, you could probably use the same principal for a treasure hunt - hide small objects around garden, 2 teams, point for first to find the object specified.