information gap Present and past tense question formation
survey--how long have you? Present-perfect & present perfect continuous
mingle activity Give advice using should. Some British English here.
Board games Lots of great board games. They are over-sized (A-4 & A-3, which is about legal-sized), but they are Word files, so you can edit them. Most have a grammar point--conditionals, present perfect, etc.
Flash cards For practicing question formation.
Handouts online has grammar practice exercises you can download and print:
Search for words, structures or collocations For high-level students. This site allows you to search a huge database of sentences used on Voice of America. For example, you can type in the word "if" and lots of real examples of conditional sentences will pop up.