In the $1, $2, $3 dollar bins at Target, they sometimes have interesting finds. I got two bug nets and bug houses for $6 total and the kids were out back catching pill bugs and picking caterpillars off the trees. Might be harder to find this time of year.

Are they into Octonauts? There's a pretty cool bath toy for 11.99 and a stuffed Barnacles for 6.59.

EDIT: How far into 3 is the little girl? Does she have similar interests to her brother? I bought Legos for my nieces (5 and 3.5 at the time) and the little one loved them. The small box of Legos costs under $15 and could be a joint gift.