Got some stacking cups from IKEA for about 60p 6 months ago - there were about 8 in the set and dd2 (now 10 months) has played with them incessantly. The Fisher Price transparent blocks with different things in that go in musical trucks and boats (can't remember what they are called) are really good, too - oh and the Tomy eggs, but they're supposed to be for 12 or 18 months or something.
Its not really a toy and people tend to love'em or hate'em but If you can get hold of a Bumbo seat she'll probably just love sitting upright. I think they're great but you only get a short time window before they start wriggling out of them so it would be a good idea to borrow one ot get a second hand one rather than pay full whack for it - I think they're about £30 quid new.
ds3 turned 6 months 2 days ago - the toys we are currently recycling for him are one of those stand/sit play centre things (we call ours davros - as thats what they look like in it), a kick and play piano for the cot (neither of the other two got the hang of this but he loves it), a kind of steering wheel that attaches to his soft seat, and various rattles and stuffed animals. He'll get the stacking cups when he can sit up. baby gyms have also still good at this age if you don't already have one.
Chunky freestanding play mirror, a ball (Lego used to do one where heads popped out and it slowed down or a Wiggly Giggly one which makes noise as rolled), stacking rings on a stick (can't think of the name !)
Both my children seemed to love the graco static bouncer that I managed to pick up for a bargain price of £25. Best buy yet! They loved being upright and active. Baby walkers seem to get a bad name these days and I didn't know if I fancied mobile babies bulldozing everything in their way!
Top toys with my 6-month old ds. Multi-coloured ball (made by Tomy) which makes a selection of noises when you tap it/bash it/throw it/drop it. dd (now 4) still plays with this so it is a good investment. Measuring spoons, either in metal or plastic - which have the added advantages of being cheap and useful in the kitchen when he has finished mauling them. Oh and crinkly animal cloth books (Mothercare). Have fun.
How quickly I forgot what it was dst played with at that age. I seem to remember the power of plain wooden spoons, good fun to wave, sook and bang on pots! That may have been a few months later but I am sure it was around this stage they played with them.
Also the stacking cups are great, as well as anything that makes a noise, is bright and shiny or has interesting textures. They did love an octupus we got from Toys R Us which had a different thing in each arm, one arm was crinkly, one was a bell and so on.