I have heard that grains are forbidden on Ekadashis. Are there any other food apart from grains that is forbidden for consumption on Ekadashis?
Ekadasi as a day to fast completely Ekadasi is a day to ideally fast completely. So, from this approach, all foods are forbidden on ekadasi, with some types of foods, and physical limitations being the exception.
EKADASYAM NA BHUNJITA KADACID API MANAVAH (HARI BHAKTI VILASA 12/15 from VISNU SMRTI) A human being should never eat on Ekadasi day.
VARAM SVA-MATRA GAMANAM VARAM GOMANSA BHAKSANAM VARAM HATYA SURAPANAM NA EKADASYANTU BHOJANAM (HARI BHAKTI VILASA 13/8 from NARADA PURANA) Worse than enjoying conception with one's own mother, eating beef, murdering, or drinking liquor is eating on Ekadasi day. Therefore one should not eat on the Ekadasi day.
For those that cannot follow the strict no eating standard There are exceptions to the no eating rule as follows
EKA BHUKTENA NAKTENA BALA VRDDHA ATURAH KSIPET PAYO MULA PHALAIR VAPI NA NIRDVAD ASIKO BHAVET (HARI BHAKTI VILASA 12/91 from MARKANDEYA PURANA) A child, an old man or a person unable to fast can take something to eat in the evening or once in the day, whether it be milk, fruit or water, but should properly follow the Ekadasi fast. One should not give up fasting on Ekadasi.
VYADHIBHIK PARIBHUTANAM PITTA ADHIKA SARIRINAM TRINSAD VARSADHIKANAN CA NAKTA-ADI PRIKALPANAM (HARI BHAKTI VILASA 12/93 from BAUDHAYANA SMRTI) Any person who is diseased, or who is dominated by bile or who has aged more than thirty years (after householder life), they are advised to eat in the evening on the Ekadasi day.
Thus, due to physical limitations, such as age, health, etc, the scriptures give some concessions. Still the most direct scriptural statements on ekadasi mention only fruits, roots, and milk products, and that one should still eat only once in the evening.
ASTA ETANY AVRTAGHNANI APO MULAM PHALAM PAYAH HAVIR BRAHMANA KAMYA CA GUROR VACANAM AUSADHAM (HARI BHAKTI VILASA 12/100 from MAHABHARATA, UDYOGA PARVA) Water, fruits, roots, milk, ghee, the request of a brahmana, the order of a spiritual master and medicine do not wane one's fast.
Modern Inclusion of Vegetables
SINCE ANNA IS MENTIONED in the scriptures, Many sampradayas today take this to mean not only grain, but to include lentils, pulses, legumes, as well.
At the same time, there is lenience towards a variety of vegetables inclusion which can differ from sampradaya to sampradaya.
For example in Gaudiya Vaisnava tradition, guidelines for strictly following observing Ekadasi would be to avoid all spices except pepper, rock salt and cumin. He should also eschew certain vegetables, such as tomatoes, cauliflower, eggplant, and leafy vegetables.
Ultimately Ekadasi is meant to develop bhakti for Bhagavan
EKA DASYAM NA BHUNJITA VRATAM ETAD DHIH VAISNAVAM (HARI BHAKTI VILASA 12/6 from AGNI PURANA) Full eating is condemned on Ekadasi and fasting on that day is certainly very pleasing to Lord Krsna.
BRAHMANA KSATRIYA VISAM SUDRANAM CAIVA YOSITAM MOKSADAM KURVATAM BHAKTYA VISNOH PRIYATARAM DVIJAH (HARI BHAKTI VILASA 12/7 from BRHAN NARADIYA PURANA) Oh brahmanas, those of brahmanas, ksatriyas, vaisyas and sudras, whoever one may be, who fast on Ekadasi with full devotion, will certainly achieve liberation.
It is recommended for Saivites and Vaisnavas
(HARI BHAKTI VILASA 12/78 from VISNU DHARMOTTARA) Whether one is a Vaisnava or a Saivite, everyone should fast on Ekadasi day.